diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/MachineNameWidget.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/MachineNameWidget.php
index c6c5716..f1a40d3 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/MachineNameWidget.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/MachineNameWidget.php
@@ -71,9 +71,6 @@ public static function defaultSettings() {
     return [
       'source_field' => '',
       'disable_on_edit' => TRUE,
-      // The 'exists' property is not configurable in the UI but it can be set
-      // manually in code.
-      'exists' => '',
       'replace_pattern' => '[^a-z0-9_]+',
       'replace' => '_',
     ] + parent::defaultSettings();
@@ -161,7 +158,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
       '#source_field' => $this->getSetting('source_field'),
       '#process' => array_merge([[get_class($this), 'processMachineNameSource']], $element_info['#process']),
       '#machine_name' => [
-        'exists' => $this->getSetting('exists'),
+        'exists' => [get_called_class(), 'exists'],
         'label' => $this->fieldDefinition->getLabel(),
         'replace_pattern' => $this->getSetting('replace_pattern'),
         'replace' => $this->getSetting('replace'),
@@ -193,4 +190,35 @@ public static function processMachineNameSource($element, FormStateInterface $fo
     return $element;
   }
 
+  /**
+   * Determines if the source field value already exists.
+   *
+   * @param mixed $value
+   *   The value entered into the machine name widget.
+   * @param array $element
+   *   The form element.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   *
+   * @return bool
+   *   TRUE if the source field value already exists, FALSE otherwise.
+   */
+  public static function exists($value, array $element, FormStateInterface $form_state) {
+    return (bool) \Drupal::entityTypeManager()
+      ->getStorage($form_state->getFormObject()->getEntity()->getEntityTypeId())
+      ->getQuery()
+      ->condition(reset($element['#array_parents']), $value, '=')
+      ->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function isApplicable(FieldDefinitionInterface $field_definition) {
+    // This widget is available only to entity identifier fields.
+    $entity_type = \Drupal::entityTypeManager()->getDefinition($field_definition->getTargetEntityTypeId());
+
+    return $field_definition->getName() === $entity_type->getKey('id');
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Render/Element/MachineName.php b/core/lib/Drupal/Core/Render/Element/MachineName.php
index 4179207..ace452b 100644
--- a/core/lib/Drupal/Core/Render/Element/MachineName.php
+++ b/core/lib/Drupal/Core/Render/Element/MachineName.php
@@ -247,7 +247,7 @@ public static function validateMachineName(&$element, FormStateInterface $form_s
     }
 
     // Verify that the machine name is unique.
-    if ($element['#default_value'] !== $element['#value'] && !empty($element['#machine_name']['exists'])) {
+    if ($element['#default_value'] !== $element['#value']) {
       $function = $element['#machine_name']['exists'];
       if (call_user_func($function, $element['#value'], $element, $form_state)) {
         $form_state->setError($element, t('The machine-readable name is already in use. It must be unique.'));
diff --git a/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestStringId.php b/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestStringId.php
index 85a5e66..05c322e 100644
--- a/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestStringId.php
+++ b/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestStringId.php
@@ -50,7 +50,12 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
       ->setReadOnly(TRUE)
       // In order to work around the InnoDB 191 character limit on utf8mb4
       // primary keys, we set the character set for the field to ASCII.
-      ->setSetting('is_ascii', TRUE);
+      ->setSetting('is_ascii', TRUE)
+      ->setDisplayConfigurable('form', TRUE);
+
+    // Make the label field configurable in the UI.
+    $fields['name']->setDisplayConfigurable('form', TRUE);
+
     return $fields;
   }
 
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Core/Field/Widget/MachineNameWidgetTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Core/Field/Widget/MachineNameWidgetTest.php
index acc085b..e7313f2 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/Core/Field/Widget/MachineNameWidgetTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Core/Field/Widget/MachineNameWidgetTest.php
@@ -2,9 +2,7 @@
 
 namespace Drupal\FunctionalJavascriptTests\Core\Field\Widget;
 
-use Drupal\entity_test\Entity\EntityTest;
-use Drupal\field\Entity\FieldConfig;
-use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\entity_test\Entity\EntityTestStringId;
 use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
 
 /**
@@ -25,50 +23,13 @@ class MachineNameWidgetTest extends JavascriptTestBase {
   public static $modules = ['entity_test', 'field', 'field_ui', 'node'];
 
   /**
-   * A field storage to use as a 'source' field for the machine name widget.
-   *
-   * @var \Drupal\field\Entity\FieldStorageConfig
-   */
-  protected $testFieldSource;
-
-  /**
-   * A field storage to use with the machine name widget.
-   *
-   * @var \Drupal\field\Entity\FieldStorageConfig
-   */
-  protected $testField;
-
-  /**
    * {@inheritdoc}
    */
   protected function setUp() {
     parent::setUp();
 
-    // Add two string fields to the entity_test entity type.
-    $this->testFieldSource = FieldStorageConfig::create([
-      'field_name' => 'test_field_source',
-      'entity_type' => 'entity_test',
-      'type' => 'string',
-    ]);
-    $this->testFieldSource->save();
-    FieldConfig::create([
-      'field_storage' => $this->testFieldSource,
-      'bundle' => 'entity_test',
-    ])->save();
-
-    $this->testField = FieldStorageConfig::create([
-      'field_name' => 'test_field',
-      'entity_type' => 'entity_test',
-      'type' => 'string',
-    ]);
-    $this->testField->save();
-    FieldConfig::create([
-      'field_storage' => $this->testField,
-      'bundle' => 'entity_test',
-    ])->save();
-
     // Create a web user.
-    $this->drupalLogin($this->drupalCreateUser(['view test entity', 'administer entity_test content', 'administer entity_test form display']));
+    $this->drupalLogin($this->drupalCreateUser(['view test entity', 'administer entity_test content', 'administer entity_test_string_id form display']));
   }
 
   /**
@@ -78,16 +39,23 @@ public function testMachineNameWidget() {
     $assert_session = $this->assertSession();
     $page = $this->getSession()->getPage();
 
+    // First, make sure that both the ID and the label fields are initially
+    // hidden in the form display.
+    entity_get_form_display('entity_test_string_id', 'entity_test_string_id', 'default')
+      ->removeComponent('id')
+      ->removeComponent('name')
+      ->save();
+
     // Go to the entity add form and check that the test fields are not
     // displayed because they are not configured in the form display yet.
-    $this->drupalGet('/entity_test/add');
-    $assert_session->fieldNotExists('test_field[0][value]');
-    $assert_session->fieldNotExists('test_field_source[0][value]');
+    $this->drupalGet('/entity_test_string_id/add');
+    $assert_session->fieldNotExists('id[0][value]');
+    $assert_session->fieldNotExists('name[0][value]');
 
     // Configure the test field to use the machine name widget with no initial
     // settings.
-    entity_get_form_display('entity_test', 'entity_test', 'default')
-      ->setComponent($this->testField->getName(), [
+    entity_get_form_display('entity_test_string_id', 'entity_test_string_id', 'default')
+      ->setComponent('id', [
         'type' => 'machine_name',
         'weight' => 5,
       ])
@@ -95,46 +63,53 @@ public function testMachineNameWidget() {
 
     // Check that the widget displays an "error" summary when it has missing or
     // broken settings.
-    $this->drupalGet('/entity_test/structure/entity_test/form-display');
+    $this->drupalGet('/entity_test_string_id/structure/entity_test_string_id/form-display');
     $assert_session->pageTextContains('Missing configuration.');
 
     // Check that test field is configured in the form display while the source
     // field is not.
-    $this->assertTrue($assert_session->optionExists('fields[test_field][region]', 'content')->isSelected());
-    $this->assertTrue($assert_session->optionExists('fields[test_field_source][region]', 'hidden')->isSelected());
+    $this->assertTrue($assert_session->optionExists('fields[id][region]', 'content')->isSelected());
+    $this->assertTrue($assert_session->optionExists('fields[name][region]', 'hidden')->isSelected());
 
     // Open the widget settings form and check that a field which is not present
     // in the form display can not be selected as a 'source_field'.
-    $page->pressButton('test_field_settings_edit');
-    $assert_session->waitForField('fields[test_field][settings_edit_form][settings][source_field]');
-    $assert_session->optionNotExists('fields[test_field][settings_edit_form][settings][source_field]', 'test_field_source');
+    $page->pressButton('id_settings_edit');
+    $assert_session->waitForField('fields[id][settings_edit_form][settings][source_field]');
+    $assert_session->optionNotExists('fields[id][settings_edit_form][settings][source_field]', 'name');
 
     // Go to the entity add form and check that the test field is not displayed,
     // even when it is enabled in the form display, because of missing settings.
-    $this->drupalGet('/entity_test/add');
-    $assert_session->fieldNotExists('test_field[0][value]');
-    $assert_session->fieldNotExists('test_field_source[0][value]');
+    $this->drupalGet('/entity_test_string_id/add');
+    $assert_session->fieldNotExists('id[0][value]');
+    $assert_session->fieldNotExists('name[0][value]');
 
     // Enable the 'source' field in the entity form display and configure the
     // test field to use it.
-    entity_get_form_display('entity_test', 'entity_test', 'default')
-      ->setComponent($this->testFieldSource->getName(), [
+    entity_get_form_display('entity_test_string_id', 'entity_test_string_id', 'default')
+      ->setComponent('name', [
         'type' => 'string_textfield',
         'weight' => 10,
       ])
       ->save();
 
-    // Go to the form display and configure the test field to use the newly
-    // enabled 'source' field as the machine name source.
-    $this->drupalGet('/entity_test/structure/entity_test/form-display');
-
-    $page->pressButton('test_field_settings_edit');
-    $assert_session->waitForField('fields[test_field][settings_edit_form][settings][source_field]');
-    $page->selectFieldOption('fields[test_field][settings_edit_form][settings][source_field]', 'test_field_source');
-    $page->pressButton('test_field_plugin_settings_update');
+    // Go to the form display and check that the machine name widget is only
+    // available for the ID field.
+    $this->drupalGet('/entity_test_string_id/structure/entity_test_string_id/form-display');
+    $assert_session->optionExists('edit-fields-name-type', 'string_textfield');
+    $assert_session->optionNotExists('edit-fields-name-type', 'machine_name');
+
+    $assert_session->optionExists('edit-fields-id-type', 'string_textfield');
+    $assert_session->optionExists('edit-fields-id-type', 'machine_name');
+
+    // Configure the test field to use the newly enabled 'source' field as the
+    // machine name source.
+    $page->pressButton('id_settings_edit');
+    $assert_session->waitForField('fields[id][settings_edit_form][settings][source_field]');
+    $page->selectFieldOption('fields[id][settings_edit_form][settings][source_field]', 'name');
+    $page->pressButton('id_plugin_settings_update');
     $assert_session->assertWaitOnAjaxRequest();
 
-    $assert_session->pageTextContains('Source field: test_field_source');
+    $assert_session->pageTextContains('Source field: Name');
     $assert_session->pageTextContains('Replace pattern: [^a-z0-9_]+');
     $assert_session->pageTextContains('Replace character: _');
 
@@ -145,26 +120,26 @@ public function testMachineNameWidget() {
     // displayed, even when it has a source field configured, because the weight
     // of the source field in the form display is higher than the weight of the
     // test field.
-    $this->drupalGet('/entity_test/add');
-    $assert_session->fieldNotExists('test_field[0][value]');
-    $assert_session->fieldExists('test_field_source[0][value]');
+    $this->drupalGet('/entity_test_string_id/add');
+    $assert_session->fieldNotExists('id[0][value]');
+    $assert_session->fieldExists('name[0][value]');
 
     // Configure the source field to have a lower weight than the test field,
     // which will finally make it appear in the entity form.
-    entity_get_form_display('entity_test', 'entity_test', 'default')
-      ->setComponent($this->testFieldSource->getName(), [
+    entity_get_form_display('entity_test_string_id', 'entity_test_string_id', 'default')
+      ->setComponent('name', [
         'type' => 'string_textfield',
         'weight' => 4,
       ])
       ->save();
 
-    $this->drupalGet('/entity_test/add');
-    $test_source_field = $page->findField('test_field_source[0][value]');
-    $test_field = $page->findField('test_field[0][value]');
-    $test_field_machine_name_value = $page->find('css', '#edit-test-field-source-0-value-machine-name-suffix .machine-name-value');
+    $this->drupalGet('/entity_test_string_id/add');
+    $test_source_field = $page->findField('name[0][value]');
+    $id = $page->findField('id[0][value]');
+    $id_machine_name_value = $page->find('css', '#edit-name-0-value-machine-name-suffix .machine-name-value');
     $this->assertNotEmpty($test_source_field);
-    $this->assertNotEmpty($test_field);
-    $this->assertNotEmpty($test_field_machine_name_value, 'Test field with the machine name widget has been initialized.');
+    $this->assertNotEmpty($id);
+    $this->assertNotEmpty($id_machine_name_value, 'Test field with the machine name widget has been initialized.');
 
     $test_values = [
       'input' => 'Test value !0-9@',
@@ -173,36 +148,36 @@ public function testMachineNameWidget() {
     $test_source_field->setValue($test_values['input']);
 
     // Wait the set timeout for fetching the machine name.
-    $this->assertJsCondition('jQuery("#edit-test-field-source-0-value-machine-name-suffix .machine-name-value").html() == "' . $test_values['expected'] . '"');
+    $this->assertJsCondition('jQuery("#edit-name-0-value-machine-name-suffix .machine-name-value").html() == "' . $test_values['expected'] . '"');
 
     // Validate the generated machine name.
-    $this->assertEquals($test_values['expected'], $test_field_machine_name_value->getHtml());
+    $this->assertEquals($test_values['expected'], $id_machine_name_value->getHtml());
 
     // Submit the entity form.
     $this->submitForm([], 'Save');
 
     // Load the entity and check that machine name value that was saved is
     // correct.
-    $entity = EntityTest::load(1);
-    $this->assertSame($test_values['input'], $entity->test_field_source->value);
-    $this->assertSame($test_values['expected'], $entity->test_field->value);
+    $entity = EntityTestStringId::load('test_value_0_9_');
+    $this->assertSame($test_values['input'], $entity->name->value);
+    $this->assertSame($test_values['expected'], $entity->id->value);
 
     // Try changing the 'replace_pattern' and 'replace' settings of the widget.
-    entity_get_form_display('entity_test', 'entity_test', 'default')
-      ->setComponent($this->testField->getName(), [
+    entity_get_form_display('entity_test_string_id', 'entity_test_string_id', 'default')
+      ->setComponent('id', [
         'type' => 'machine_name',
         'weight' => 5,
         'settings' => [
-          'source_field' => $this->testFieldSource->getName(),
+          'source_field' => 'name',
           'replace_pattern' => '[^a-z0-9-]+',
           'replace' => '-',
         ],
       ])
       ->save();
 
-    $this->drupalGet('/entity_test/add');
-    $test_source_field = $page->findField('test_field_source[0][value]');
-    $test_field_machine_name_value = $page->find('css', '#edit-test-field-source-0-value-machine-name-suffix .machine-name-value');
+    $this->drupalGet('/entity_test_string_id/add');
+    $test_source_field = $page->findField('name[0][value]');
+    $id_machine_name_value = $page->find('css', '#edit-name-0-value-machine-name-suffix .machine-name-value');
 
     $test_values = [
       'input' => 'Test value2 !0-9@',
@@ -211,19 +186,43 @@ public function testMachineNameWidget() {
     $test_source_field->setValue($test_values['input']);
 
     // Wait the set timeout for fetching the machine name.
-    $this->assertJsCondition('jQuery("#edit-test-field-source-0-value-machine-name-suffix .machine-name-value").html() == "' . $test_values['expected'] . '"');
+    $this->assertJsCondition('jQuery("#edit-name-0-value-machine-name-suffix .machine-name-value").html() == "' . $test_values['expected'] . '"');
 
     // Validate the generated machine name.
-    $this->assertEquals($test_values['expected'], $test_field_machine_name_value->getHtml());
+    $this->assertEquals($test_values['expected'], $id_machine_name_value->getHtml());
 
     // Submit the entity form.
     $this->submitForm([], 'Save');
 
     // Load the entity and check that machine name value that was saved is
     // correct.
-    $entity = EntityTest::load(2);
-    $this->assertSame($test_values['input'], $entity->test_field_source->value);
-    $this->assertSame($test_values['expected'], $entity->test_field->value);
+    $entity = EntityTestStringId::load('test-value2-0-9-');
+    $this->assertSame($test_values['input'], $entity->name->value);
+    $this->assertSame($test_values['expected'], $entity->id->value);
+
+    // Repeat the steps above in order to check that entering an existing value
+    // in the machine name widget throws an error.
+    $this->drupalGet('/entity_test_string_id/add');
+    $test_source_field = $page->findField('name[0][value]');
+    $id_machine_name_value = $page->find('css', '#edit-name-0-value-machine-name-suffix .machine-name-value');
+
+    $test_values = [
+      'input' => 'Test value2 !0-9@',
+      'expected' => 'test-value2-0-9-',
+    ];
+    $test_source_field->setValue($test_values['input']);
+
+    // Wait the set timeout for fetching the machine name.
+    $this->assertJsCondition('jQuery("#edit-name-0-value-machine-name-suffix .machine-name-value").html() == "' . $test_values['expected'] . '"');
+
+    // Validate the generated machine name.
+    $this->assertEquals($test_values['expected'], $id_machine_name_value->getHtml());
+
+    // Submit the entity form.
+    $this->submitForm([], 'Save');
+
+    // Check that a form-level error has been thrown.
+    $assert_session->pageTextContains('The machine-readable name is already in use. It must be unique.');
   }
 
 }