diff --git a/plugins/FeedsUserProcessor.inc b/plugins/FeedsUserProcessor.inc
index 912fbfa..94e70e2 100644
--- a/plugins/FeedsUserProcessor.inc
+++ b/plugins/FeedsUserProcessor.inc
@@ -105,6 +105,21 @@ class FeedsUserProcessor extends FeedsProcessor {
     if (empty($account->name) || empty($account->mail) || !valid_email_address($account->mail)) {
       throw new FeedsValidationException(t('User name missing or email not valid.'));
     }
+
+    // Check when an user ID gets set or changed during processing if that user
+    // ID is not already in use.
+    if (!empty($account->uid)) {
+      $is_new = !empty($account->feeds_item->is_new);
+      $different = !empty($account->feeds_item->entity_id) && $account->feeds_item->entity_id != $account->uid;
+      if ($is_new || $different) {
+        $exists = entity_load_unchanged('user', $account->uid);
+        if ($exists) {
+          throw new FeedsValidationException(t('Could not update user ID to @uid since that ID is already in use.', array(
+            '@uid' => $account->uid,
+          )));
+        }
+      }
+    }
   }
 
   /**
@@ -122,6 +137,17 @@ class FeedsUserProcessor extends FeedsProcessor {
       unset($edit['pass']);
     }
 
+    // Check if the user ID changed when updating users.
+    if (!empty($account->feeds_item->entity_id) && $account->feeds_item->entity_id != $account->uid) {
+      // The user ID of the existing user is different. Try to update the user ID.
+      db_update('users')
+        ->fields(array(
+          'uid' => $account->uid,
+        ))
+        ->condition('uid', $account->feeds_item->entity_id)
+        ->execute();
+    }
+
     user_save($account, $edit);
 
     // If an encrypted password was given, directly set this in the database.
diff --git a/tests/feeds_processor_user.test b/tests/feeds_processor_user.test
index 7b31af7..f4a6e42 100644
--- a/tests/feeds_processor_user.test
+++ b/tests/feeds_processor_user.test
@@ -205,6 +205,61 @@ class FeedsCSVtoUsersTest extends FeedsWebTestCase {
   }
 
   /**
+   * Tests if user ID's can be changed using the user ID target.
+   *
+   * Also checks if a clear error is reported when trying to change the
+   * user ID to something that is already in use.
+   */
+  public function testUidUpdating() {
+    // Set to update existing users.
+    $this->setSettings('user_import', 'FeedsUserProcessor', array('update_existing' => FEEDS_UPDATE_EXISTING));
+
+    // Add mapping to user ID, but do not mark target as unique.
+    $this->addMappings('user_import', array(
+      4 => array(
+        'source' => 'uid',
+        'target' => 'uid',
+      ),
+    ));
+
+    // Create an account which user ID should be updated.
+    user_save(drupal_anonymous_user(), array(
+      'uid' => 54,
+      'name' => 'Morticia',
+      'mail' => 'morticia@example.com',
+      'pass' => 'mort',
+      'status' => 1,
+    ));
+
+    // Create account with uid 202. Importing an other account with uid 202
+    // should fail.
+    user_save(drupal_anonymous_user(), array(
+      'uid' => 202,
+      'name' => 'Joe',
+      'mail' => 'joe@example.com',
+      'pass' => 'joe',
+      'status' => 1,
+    ));
+
+    // Import CSV file.
+    $this->importFile('user_import', $this->absolutePath() . '/tests/feeds/users.csv');
+    $this->assertText('Created 1 user');
+    $this->assertText('Updated 1 user');
+    $this->assertText('Failed importing 3 users.');
+    $this->assertText('Could not update user ID to 202 since that ID is already in use.');
+
+    // Assert Morticia's user ID got updated.
+    $account = user_load_by_name('Morticia');
+    $this->assertEqual(201, $account->uid, 'Morticia now got user ID 201.');
+    // Assert that Fester failed to import.
+    $this->assertFalse(user_load_by_name('Fester'), 'The account for Fester was not imported.');
+    // Assert that user 202 did not change.
+    $account = user_load(202);
+    $this->assertEqual('Joe', $account->name, 'The user name of account 202 is still Joe.');
+    $this->assertEqual('joe@example.com', $account->mail, 'The mail address of account 202 is still joe@example.com.');
+  }
+
+  /**
    * Tests mapping to role without automatically creating new roles.
    */
   public function testRoleTargetWithoutRoleCreation() {