diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 70f75d6..ba07282 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -690,9 +690,10 @@ function simpletest_clean_database() {
  *   to be of the following pattern: sites/simpletest/12345678. If ommited, all
  *   the test sites' directories found in sites/simpletest will be removed.
  *
- * @return bool
- *   TRUE if directory is removed, FALSE otherwise. Additionally watchdog
- *   messages will be emitted if any path attempted to remove fails.
+ * @return null|bool
+ *   Returns TRUE if all attempted to remove directories are removed, FALSE if
+ *   at least one directory was not removed successfully, NULL if there is
+ *   nothing to remove or the directory is not eligible for removing.
  *
  * @see file_unmanaged_delete()
  * @see http://php.net/manual/en/function.unlink.php
@@ -700,7 +701,7 @@ function simpletest_clean_database() {
 function simpletest_clean_temporary_directories($directory = NULL) {
   $directories = [];
   $count = 0;
-  $result = FALSE;
+  $result = NULL;
   $simpletest_root = \Drupal::root() . '/sites/simpletest/';
 
   if (is_dir($simpletest_root)) {
@@ -718,8 +719,7 @@ function simpletest_clean_temporary_directories($directory = NULL) {
       $directories = scandir($simpletest_root);
     }
     else {
-      $logger = \Drupal::logger('Testing');
-      $logger->error('The %path is not eligible for removing in %func().', ['%path' => $path, '%func' => __FUNCTION__]);
+      drupal_set_message(t('The %path is not eligible for removing in %func().', ['%path' => $path, '%func' => __FUNCTION__]));
     }
 
     foreach ($directories as $file) {
@@ -729,25 +729,25 @@ function simpletest_clean_temporary_directories($directory = NULL) {
         // runner, we can make read-only files writable again. If not, chmod
         // will fail while the file deletion still works if file permissions
         // have been configured correctly. Thus, we ignore any chmod errors.
-        $result = file_unmanaged_delete_recursive($path, function ($any_path) {
-          // @see http://php.net/manual/en/function.chmod.php
+        $deleted = file_unmanaged_delete_recursive($path, function ($any_path) {
           @chmod($any_path, 0700);
         });
-        $count++;
+        $result = $result === FALSE ? $result : $deleted;
+        if ($deleted) {
+          $count++;
+        }
+        else {
+          drupal_set_message(t('This directory is failed to be removed: @path.', ['@path' => $path]));
+        }
       }
     }
   }
 
-  // To prevent some tests failure do not display any messages if this function
-  // was called from within a test instead of using "Clean environment" button.
-  // @see https://www.drupal.org/node/2745123#comment-11837053
-  if ($directory === NULL) {
-    if ($count > 0) {
-      drupal_set_message(\Drupal::translation()->formatPlural($count, 'Removed 1 temporary directory.', 'Removed @count temporary directories.'));
-    }
-    else {
-      drupal_set_message(t('No temporary directories to remove.'));
-    }
+  if ($count > 0) {
+    drupal_set_message(\Drupal::translation()->formatPlural($count, 'Removed 1 temporary directory.', 'Removed @count temporary directories.'));
+  }
+  else {
+    drupal_set_message(t('No temporary directories to remove.'));
   }
 
   return $result;
diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index b6e2c3e..14d7909 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1231,8 +1231,8 @@ private function restoreEnvironment() {
     $this->container = $this->originalContainer;
     \Drupal::setContainer($this->originalContainer);
 
-    // Delete the current test site directory.
-    simpletest_clean_temporary_directories($this->siteDirectory);
+    // Delete test site directory.
+    file_unmanaged_delete_recursive($this->siteDirectory, array($this, 'filePreDeleteCallback'));
 
     // Restore original database connection.
     Database::removeConnection('default');
@@ -1393,6 +1393,21 @@ public static function generatePermutations($parameters) {
   }
 
   /**
+   * Ensures test files are deletable within file_unmanaged_delete_recursive().
+   *
+   * Some tests chmod generated files to be read only. During
+   * TestBase::restoreEnvironment() and other cleanup operations, these files
+   * need to get deleted too.
+   */
+  public static function filePreDeleteCallback($path) {
+    // When the webserver runs with the same system user as the test runner, we
+    // can make read-only files writable again. If not, chmod will fail while
+    // the file deletion still works if file permissions have been configured
+    // correctly. Thus, we ignore any problems while running chmod.
+    @chmod($path, 0700);
+  }
+
+  /**
    * Configuration accessor for tests. Returns non-overridden configuration.
    *
    * @param $name
diff --git a/core/modules/simpletest/src/Tests/UiCleanTemporaryDirectoriesTest.php b/core/modules/simpletest/src/Tests/UiCleanTemporaryDirectoriesTest.php
new file mode 100644
index 0000000..05235f4
--- /dev/null
+++ b/core/modules/simpletest/src/Tests/UiCleanTemporaryDirectoriesTest.php
@@ -0,0 +1,70 @@
+<?php
+
+namespace Drupal\simpletest\Tests;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests removing the Simpletest temporary directories through UI.
+ *
+ * @group simpletest
+ */
+class UiCleanTemporaryDirectoriesTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('simpletest', 'testing_page_test',);
+
+  protected function setUp() {
+    parent::setUp();
+    // Create and log in an admin user.
+    $this->drupalLogin($this->drupalCreateUser(array('administer unit tests')));
+  }
+
+  /**
+   * Tests removing a temporary directory through the UI.
+   */
+  public function testCleanTemporaryDirectories() {
+    $simpletest = \Drupal::root() . '/sites/simpletest';
+    $test_site_directory = basename($this->siteDirectory);
+    $clean_environment = t('Clean environment');
+    $this->drupalGet('admin/config/development/testing');
+
+    $this->assertFieldByXPath('//input', $clean_environment, 'Displayed the "Clean environment" button.');
+    $this->assertNull(simpletest_clean_temporary_directories('sites/simpletest/test_site_directory'), 'The sites/simpletest/test_site_directory folder does not exist.');
+
+    $this->recurse_copy_test_site_directory("$simpletest/$test_site_directory", "$simpletest/test_site_directory");
+
+    $this->assertTrue(is_dir("$simpletest/test_site_directory"), 'The test site directory is copied to the sites/simpletest/test_site_directory folder.');
+
+    $this->drupalPostForm(NULL, [], $clean_environment);
+
+    $this->assertText(t('Removed @count temporary directory.', ['@count' => 1]), 'Displayed a message: "Removed 1 temporary directory.".');
+    $this->assertNoText(t('This directory is failed to be removed:'), 'The message "This directory is failed to be removed:" is not displayed.');
+    $this->assertNull(simpletest_clean_temporary_directories('sites/simpletest/test_site_directory'), 'The sites/simpletest/test_site_directory folder does not exist any more.');
+  }
+
+  /**
+   * Creates the exact copy of the test site temporary directory.
+   */
+  protected function recurse_copy_test_site_directory($source, $destination) {
+    $directory = opendir($source);
+    mkdir($destination);
+
+    while(($file = readdir($directory)) !== FALSE) {
+      if ($file != '.' && $file != '..') {
+        if (is_dir("$source/$file")) {
+          $this->recurse_copy_test_site_directory("$source/$file", "$destination/$file");
+        }
+        else {
+          copy("$source/$file", "$destination/$file");
+        }
+      }
+    }
+    closedir($directory);
+  }
+
+}
diff --git a/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.info.yml b/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.info.yml
new file mode 100644
index 0000000..33773ba
--- /dev/null
+++ b/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.info.yml
@@ -0,0 +1,6 @@
+name: 'Testing page test'
+type: module
+description: 'Provides customizations for the simpletest testing page.'
+package: Testing
+version: VERSION
+core: 8.x
diff --git a/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.module b/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.module
new file mode 100644
index 0000000..da2d82b
--- /dev/null
+++ b/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.module
@@ -0,0 +1,22 @@
+<?php
+
+/**
+ * @file
+ * Defines overrides for the simpletest testing page.
+ */
+
+/**
+ * Implements hook_form_alter().
+ */
+function testing_page_test_form_alter(&$form, $form_state, $form_id) {
+  if ($form_id == 'simpletest_test_form') {
+    $form['clean']['op']['#submit'] = ['testing_page_test_submit'];
+  }
+}
+
+/**
+ * Removes test site directory when the 'Clean environment' button is pressed.
+ */
+function testing_page_test_submit() {
+  simpletest_clean_temporary_directories('sites/simpletest/test_site_directory');
+}
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index b1ae138..9e832a5 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -911,9 +911,11 @@ function simpletest_script_cleanup($test_id, $test_class, $exitcode) {
         $messages[] = $errors;
       }
     }
-    // Delete the current test site directory leaving alone other tests that
-    // are potentially running concurrently.
-    simpletest_clean_temporary_directories($test_db->getTestSitePath());
+    // Delete the test site directory.
+    // simpletest_clean_temporary_directories() cannot be used here, since it
+    // would also delete file directories of other tests that are potentially
+    // running concurrently.
+    file_unmanaged_delete_recursive($test_directory, array('Drupal\simpletest\TestBase', 'filePreDeleteCallback'));
     $messages[] = "- Removed test site directory.";
   }
 
diff --git a/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
index 554d8f1..d4968f4 100644
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -457,6 +457,24 @@ protected function setUp() {
   }
 
   /**
+   * Ensures test files are deletable within file_unmanaged_delete_recursive().
+   *
+   * Some tests chmod generated files to be read only. During
+   * BrowserTestBase::cleanupEnvironment() and other cleanup operations,
+   * these files need to get deleted too.
+   *
+   * @param string $path
+   *   The file path.
+   */
+  public static function filePreDeleteCallback($path) {
+    // When the webserver runs with the same system user as phpunit, we can
+    // make read-only files writable again. If not, chmod will fail while the
+    // file deletion still works if file permissions have been configured
+    // correctly. Thus, we ignore any problems while running chmod.
+    @chmod($path, 0700);
+  }
+
+  /**
    * Clean up the Simpletest environment.
    */
   protected function cleanupEnvironment() {
@@ -474,8 +492,8 @@ protected function cleanupEnvironment() {
       }
     }
 
-    // Delete the current test site directory.
-    simpletest_clean_temporary_directories($this->siteDirectory);
+    // Delete test site directory.
+    file_unmanaged_delete_recursive($this->siteDirectory, array($this, 'filePreDeleteCallback'));
   }
 
   /**