diff --git a/entity_translation.module b/entity_translation.module
index 053d39d..e1104c4 100644
--- a/entity_translation.module
+++ b/entity_translation.module
@@ -193,6 +193,7 @@ function entity_translation_menu() {
   return $items;
 }
 
+
 /**
  * Implements hook_admin_paths().
  */
@@ -465,11 +466,165 @@ function entity_translation_edit_form_submit($form, &$form_state) {
 function entity_translation_form_field_ui_field_settings_form_alter(&$form, $form_state) {
   $instance = $form_state['build_info']['args'][0];
   $field = field_info_field($instance['field_name']);
+
   $form['field']['translatable'] = array(
     '#type' => 'checkbox',
     '#title' => t('Users may translate this field.'),
     '#default_value' => $field['translatable'],
   );
+
+  $form['field']['warning'] = array(
+    '#type' => 'container',
+    '#states' => array(
+      'visible' => array(
+        ':input[name="field[translatable]"]' => array('checked' => !$field['translatable']),
+      ),
+    ),
+  );
+
+  $warning = t('By submitting this form you will trigger a batch operation.');
+  if ($field['translatable']) {
+    // Once the confirm page is finished, we can drop the extra warning text.
+    $warning .= ' ' . t('<em>All translations of this field will be deleted.</em> Please backup your data before proceeding.');
+  }
+  $form['field']['warning']['warning'] = array(
+    '#markup' => '<div class="messages warning">' . $warning . '</div>',
+  );
+
+  // If a field is untranslatable, it can have no data except under LANGUAGE_NONE,
+  // Thus we need a field to be translatable before we convert data to the entity
+  // language. Conversely we need to convert data back to LANGUAGE_NONE before
+  // making a field untranslatable lest we lose information.
+  if (!isset($form['#submit'])) {
+    $form['#submit'] = array();
+  }
+  if ($field['translatable']) {
+    array_unshift($form['#submit'], 'entity_translation_form_translatable_submit');
+  }
+  else {
+    $form['#submit'][] = 'entity_translation_form_translatable_submit';
+  }
+}
+
+/**
+ * Submit handler for the field settings form.
+ *
+ * This submit handler maintains consistency between the translatability of an
+ * entity and the language under which the field data is stored. When a field is
+ * marked as translatable, all the data in $entity->{field_name}[LANGUAGE_NONE]
+ * is moved to $entity->{field_name}[$entity_language]. When a field is marked
+ * as untranslatable the opposite process occurs. Note that marking a field as
+ * untranslatable will cause all of its translations to be permanently removed,
+ * with the exception of the one corresponding to the entity language.
+ */
+function entity_translation_form_translatable_submit($form, &$form_state) {
+  $translatable = $form_state['values']['field']['translatable'];
+  $previous = $form['field']['translatable']['#default_value'];
+
+  if ($previous == $translatable) {
+    // Nothing to do.
+    return;
+  }
+
+  $field_name = $form_state['values']['field']['field_name'];
+
+  $batch = array(
+    'title' => t('Converting'),
+    'operations' => array(
+      array('entity_translation_translatable_batch', array($translatable, $field_name)),
+    ),
+    'finished' => 'entity_translation_translatable_batch_done',
+  );
+
+  batch_set($batch);
+}
+
+/**
+ * Batch operation. Switch the translatability of the given field.
+ */
+function entity_translation_translatable_batch($make_translatable, $field_name, &$context) {
+  if (empty($context['sandbox'])) {
+    $context['sandbox']['progress'] = 0;
+
+    // How many entities will need processing?
+    $query = new EntityFieldQuery();
+    $count = $query
+      ->fieldCondition($field_name)
+      ->count()
+      ->execute();
+
+    if (intval($count) === 0) {
+      // Nothing to do.
+      $context['finished'] = 1;
+      return;
+    }
+
+    $context['sandbox']['max'] = $count;
+  }
+
+  // Number of entities to be processed for each step.
+  $limit = 10;
+
+  $query = new EntityFieldQuery();
+  $result = $query
+    ->fieldCondition($field_name)
+    ->range($context['sandbox']['progress'], $limit)
+    ->execute();
+
+  foreach ($result as $entity_type => $entities) {
+    foreach (entity_load($entity_type, array_keys($entities)) as $id => $entity) {
+      $context['sandbox']['progress']++;
+      $langcode = entity_translation_get_handler($entity_type, $entity)->getLanguage();
+
+      if ($make_translatable && isset($entity->{$field_name}[LANGUAGE_NONE])) {
+        // If the field is being switched to translatable and has data for
+        // LANGUAGE_NONE then we need to move the data to the right language.
+        $entity->{$field_name}[$langcode] = $entity->{$field_name}[LANGUAGE_NONE];
+        $entity->{$field_name}[LANGUAGE_NONE] = array();
+      }
+      elseif (!$make_translatable && isset($entity->{$field_name}[$langcode])) {
+        // The field has been marked untranslatable and has data in the entity
+        // language: we need to move it to LANGUAGE_NONE and drop the other
+        // translations.
+        $data = $entity->{$field_name}[$langcode];
+        // Remove all translations of the field data.
+        foreach ($entity->{$field_name} as $langcode => $items) {
+          $entity->{$field_name}[$langcode] = array();
+        }
+        // Move the original field data to LANGUAGE_NONE
+        $entity->{$field_name}[LANGUAGE_NONE] = $data;
+      }
+      else {
+        // No need to save unchanged entities.
+        continue;
+      }
+
+      // Save field data.
+      field_attach_update($entity_type, $entity);
+    }
+  }
+
+  $context['finished'] = $context['sandbox']['progress']/$context['sandbox']['max'];
+
+  // Workaround for http://drupal.org/node/1300928.
+  // If the batch op runs first, then by the time the core field submit handler
+  // gets called, its code is out of scope.
+  if ($context['finished'] >= 1) {
+    module_load_include('inc', 'field_ui', 'field_ui.admin');
+  }
+}
+
+/**
+ * Check the exit status of the batch operation, and possibly resubmit the changed translatable status.
+ */
+function entity_translation_translatable_batch_done($success, $results, $operations) {
+  if ($success) {
+    drupal_set_message(t("Data successfully converted."));
+  }
+  else {
+    // TODO: Do something about this case.
+    drupal_set_message(t("Something went wrong when converting the data. Some nodes may appear to have lost fields."));
+  }
 }
 
 /**