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 f6e2940625..6412f4c8e6 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/MachineNameWidget.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/MachineNameWidget.php
@@ -160,7 +160,11 @@ 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' => [get_called_class(), 'exists'],
+        // We don't need the default form-level validation because we enforce
+        // the 'UniqueField' constraint on the field that uses this widget.
+        'exists' => function () {
+          return FALSE;
+        },
         'label' => $this->fieldDefinition->getLabel(),
         'replace_pattern' => $this->getSetting('replace_pattern'),
         'replace' => $this->getSetting('replace'),
@@ -192,35 +196,14 @@ public static function processMachineNameSource($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 $form_state) {
-    return (bool) \Drupal::entityTypeManager()
-      ->getStorage($element['#entity_type_id'])
-      ->getQuery()
-      ->condition($element['#field_name'], $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');
+    // This widget is only available to fields that have a 'UniqueField'
+    // constraint.
+    $constraints = $field_definition->getConstraints();
+    return isset($constraints['UniqueField']);
   }
 
 }
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 05c322e967..042b3b781e 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
@@ -51,6 +51,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
       // 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)
+      ->addConstraint('UniqueField', [])
       ->setDisplayConfigurable('form', TRUE);
 
     // Make the label field configurable in the UI.
diff --git a/core/modules/workspaces/src/Entity/Workspace.php b/core/modules/workspaces/src/Entity/Workspace.php
index 149bf0947a..1903fc9f73 100644
--- a/core/modules/workspaces/src/Entity/Workspace.php
+++ b/core/modules/workspaces/src/Entity/Workspace.php
@@ -79,8 +79,8 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
       ->setDescription(new TranslatableMarkup('The workspace ID.'))
       ->setSetting('max_length', 128)
       ->setRequired(TRUE)
-      ->addConstraint('UniqueField')
-      ->addConstraint('DeletedWorkspace')
+      ->addConstraint('UniqueField', [])
+      ->addConstraint('DeletedWorkspace', [])
       ->addPropertyConstraints('value', ['Regex' => ['pattern' => '/^[a-z0-9_]+$/']])
       ->setDisplayOptions('form', [
         'type' => 'machine_name',
diff --git a/core/modules/workspaces/tests/src/Functional/WorkspaceTest.php b/core/modules/workspaces/tests/src/Functional/WorkspaceTest.php
index 36f62d1c05..edbec2b1f8 100644
--- a/core/modules/workspaces/tests/src/Functional/WorkspaceTest.php
+++ b/core/modules/workspaces/tests/src/Functional/WorkspaceTest.php
@@ -52,23 +52,24 @@ public function setUp() {
   }
 
   /**
-   * Test creating a workspace with special characters.
+   * Test creating a workspace.
    */
-  public function testSpecialCharacters() {
+  public function testWorkspaceCreate() {
     $this->drupalLogin($this->editor1);
+    $page = $this->getSession()->getPage();
 
-    // Test a valid workspace name.
-    $this->createWorkspaceThroughUi('Workspace 1', 'a0_$()+-/');
+    // Test a valid workspace ID.
+    $workspace = $this->createWorkspaceThroughUi('Workspace 1', 'workspace_1');
+    $this->assertEquals('workspace_1', $workspace->id());
 
-    // Test and invalid workspace name.
-    $this->drupalGet('/admin/config/workflow/workspaces/add');
-    $this->assertSession()->statusCodeEquals(200);
+    // Test an invalid workspace ID.
+    $workspace = $this->createWorkspaceThroughUi('Workspace 2', 'workspace A@-');
+    $this->assertNull($workspace);
+    $this->assertTrue($page->hasContent('The machine-readable name must contain only lowercase letters, numbers, and underscores.'));
 
-    $page = $this->getSession()->getPage();
-    $page->fillField('label[0][value]', 'workspace2');
-    $page->fillField('id[0][value]', 'A!"£%^&*{}#~@?');
-    $page->findButton('Save')->click();
-    $page->hasContent("This value is not valid");
+    // Test a duplicate workspace ID.
+    $this->createWorkspaceThroughUi('Workspace 1 again', 'workspace_1');
+    $this->assertTrue($page->hasContent('A workspace with workspace id workspace_1 already exists.'));
   }
 
   /**
diff --git a/core/modules/workspaces/tests/src/Functional/WorkspaceTestUtilities.php b/core/modules/workspaces/tests/src/Functional/WorkspaceTestUtilities.php
index 43fbf5c777..67e1423569 100644
--- a/core/modules/workspaces/tests/src/Functional/WorkspaceTestUtilities.php
+++ b/core/modules/workspaces/tests/src/Functional/WorkspaceTestUtilities.php
@@ -59,8 +59,6 @@ protected function createWorkspaceThroughUi($label, $id) {
       'label[0][value]' => $label,
     ], 'Save');
 
-    $this->getSession()->getPage()->hasContent("$label ($id)");
-
     return Workspace::load($id);
   }
 
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Core/Field/Widget/MachineNameWidgetTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Core/Field/Widget/MachineNameWidgetTest.php
index 94171cef89..ef778be702 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/Core/Field/Widget/MachineNameWidgetTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Core/Field/Widget/MachineNameWidgetTest.php
@@ -222,7 +222,7 @@ public function testMachineNameWidget() {
     $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.');
+    $assert_session->pageTextContains('A test entity with string_id with id test-value2-0-9- already exists.');
   }
 
 }