diff --git a/mod/assign/lang/en/assign.php b/mod/assign/lang/en/assign.php
index 44f9f803e14..04ae870a3cd 100644
--- a/mod/assign/lang/en/assign.php
+++ b/mod/assign/lang/en/assign.php
@@ -133,6 +133,8 @@ $string['duedate'] = 'Due date';
 $string['duedate_help'] = 'This is when the assignment is due. Submissions will still be allowed after this date but any assignments submitted after this date are marked as late. To prevent submissions after a certain date - set the assignment cut off date.';
 $string['duedateno'] = 'No due date';
 $string['submissionempty'] = 'Nothing was submitted';
+$string['submissionmodified'] = 'You have existing submission data. Please leave this page and try again.';
+$string['submissionmodifiedgroup'] = 'The submission has been modified by somebody else. Please leave this page and try again.';
 $string['duedatereached'] = 'The due date for this assignment has now passed';
 $string['duedatevalidation'] = 'Due date must be after the allow submissions from date.';
 $string['editattemptfeedback'] = 'Edit the grade and feedback for attempt number {$a}.';
diff --git a/mod/assign/locallib.php b/mod/assign/locallib.php
index e510b64d93a..b1f91480c3b 100644
--- a/mod/assign/locallib.php
+++ b/mod/assign/locallib.php
@@ -5927,6 +5927,17 @@ class assign {
         } else {
             $submission = $this->get_user_submission($userid, true);
         }
+
+        // Check that no one has modified the submission since we started looking at it.
+        if (isset($data->lastmodified) && ($submission->timemodified > $data->lastmodified)) {
+            // Another user has submitted something. Notify the current user.
+            if ($submission->status !== ASSIGN_SUBMISSION_STATUS_NEW) {
+                $notices[] = $instance->teamsubmission ? get_string('submissionmodifiedgroup', 'mod_assign')
+                                                       : get_string('submissionmodified', 'mod_assign');
+                return false;
+            }
+        }
+
         if ($instance->submissiondrafts) {
             $submission->status = ASSIGN_SUBMISSION_STATUS_DRAFT;
         } else {
diff --git a/mod/assign/submission_form.php b/mod/assign/submission_form.php
index fc466697111..99f44169dd8 100644
--- a/mod/assign/submission_form.php
+++ b/mod/assign/submission_form.php
@@ -41,12 +41,22 @@ class mod_assign_submission_form extends moodleform {
      * Define this form - called by the parent constructor
      */
     public function definition() {
+        global $USER;
         $mform = $this->_form;
-
         list($assign, $data) = $this->_customdata;
 
-        $assign->add_submission_form_elements($mform, $data);
+        $instance = $assign->get_instance();
+        if ($instance->teamsubmission) {
+            $submission = $assign->get_group_submission($USER->id, 0, true);
+        } else {
+            $submission = $assign->get_user_submission($USER->id, true);
+        }
+        if ($submission) {
+            $mform->addElement('hidden', 'lastmodified', $submission->timemodified);
+            $mform->setType('lastmodified', PARAM_INT);
+        }
 
+        $assign->add_submission_form_elements($mform, $data);
         $this->add_action_buttons(true, get_string('savechanges', 'assign'));
         if ($data) {
             $this->set_data($data);