diff --git a/blocks/activity_results/tests/behat/addunsupportedactivity.feature b/blocks/activity_results/tests/behat/addunsupportedactivity.feature
index 111c96f96a9..012549ea5e5 100644
--- a/blocks/activity_results/tests/behat/addunsupportedactivity.feature
+++ b/blocks/activity_results/tests/behat/addunsupportedactivity.feature
@@ -35,6 +35,6 @@ Feature: The activity results block displays student scores
     When I follow "Test assignment"
     And I click on "Edit settings" "link" in the "Administration" "block"
     And I set the following fields to these values:
-      | id_modgrade_type | None |
+      | id_grade_modgrade_type | None |
     And I press "Save and return to course"
     Then I should see "Error: the activity selected uses a grading method that is not supported by this block." in the "Activity results" "block"
diff --git a/blocks/activity_results/tests/behat/highscoreswithscales.feature b/blocks/activity_results/tests/behat/highscoreswithscales.feature
index 1263382f84b..b5e52f875a1 100644
--- a/blocks/activity_results/tests/behat/highscoreswithscales.feature
+++ b/blocks/activity_results/tests/behat/highscoreswithscales.feature
@@ -39,8 +39,8 @@ Feature: The activity results block displays student scores as scales
       | Assignment name | Test assignment |
       | Description | Offline text |
       | assignsubmission_file_enabled | 0 |
-      | id_modgrade_type | Scale |
-      | id_modgrade_scale | My Scale |
+      | id_grade_modgrade_type | Scale |
+      | id_grade_modgrade_scale | My Scale |
     And I follow "Course 1"
     And I navigate to "Grades" node in "Course administration"
     And I turn editing mode on
diff --git a/blocks/activity_results/tests/behat/highscoreswithscalesandgroups.feature b/blocks/activity_results/tests/behat/highscoreswithscalesandgroups.feature
index 7856da599b3..d546fda0c5c 100644
--- a/blocks/activity_results/tests/behat/highscoreswithscalesandgroups.feature
+++ b/blocks/activity_results/tests/behat/highscoreswithscalesandgroups.feature
@@ -56,8 +56,8 @@ Feature: The activity results block displays student scores as scales
       | Assignment name | Test assignment |
       | Description | Offline text |
       | assignsubmission_file_enabled | 0 |
-      | id_modgrade_type | Scale |
-      | id_modgrade_scale | My Scale |
+      | id_grade_modgrade_type | Scale |
+      | id_grade_modgrade_scale | My Scale |
       | Group mode | Separate groups |
     And I follow "Course 1"
     And I navigate to "Grades" node in "Course administration"
diff --git a/blocks/activity_results/tests/behat/lowscoreswithscales.feature b/blocks/activity_results/tests/behat/lowscoreswithscales.feature
index 149e0f60f0c..00f31f6b48b 100644
--- a/blocks/activity_results/tests/behat/lowscoreswithscales.feature
+++ b/blocks/activity_results/tests/behat/lowscoreswithscales.feature
@@ -39,8 +39,8 @@ Feature: The activity results block displays student scores as scales
       | Assignment name | Test assignment |
       | Description | Offline text |
       | assignsubmission_file_enabled | 0 |
-      | id_modgrade_type | Scale |
-      | id_modgrade_scale | My Scale |
+      | id_grade_modgrade_type | Scale |
+      | id_grade_modgrade_scale | My Scale |
     And I follow "Course 1"
     And I navigate to "Grades" node in "Course administration"
     And I turn editing mode on
diff --git a/blocks/activity_results/tests/behat/lowscoreswithscalesandgroups.feature b/blocks/activity_results/tests/behat/lowscoreswithscalesandgroups.feature
index 91343398269..be5ae335e79 100644
--- a/blocks/activity_results/tests/behat/lowscoreswithscalesandgroups.feature
+++ b/blocks/activity_results/tests/behat/lowscoreswithscalesandgroups.feature
@@ -56,8 +56,8 @@ Feature: The activity results block displays student scores as scales
       | Assignment name | Test assignment |
       | Description | Offline text |
       | assignsubmission_file_enabled | 0 |
-      | id_modgrade_type | Scale |
-      | id_modgrade_scale | My Scale |
+      | id_grade_modgrade_type | Scale |
+      | id_grade_modgrade_scale | My Scale |
       | Group mode | Separate groups |
     And I follow "Course 1"
     And I navigate to "Grades" node in "Course administration"
diff --git a/lib/form/modgrade.php b/lib/form/modgrade.php
index 2d8f54fbf06..d997790b9c8 100644
--- a/lib/form/modgrade.php
+++ b/lib/form/modgrade.php
@@ -88,15 +88,15 @@ class MoodleQuickForm_modgrade extends MoodleQuickForm_group{
         $langscale = get_string('modgradetypescale', 'grades');
         $scaleselect = @MoodleQuickForm::createElement('select', 'modgrade_scale', $langscale, $scales, $attributes);
         $scaleselect->setHiddenLabel = false;
-        $scaleselect->_generateId();
-        $scaleselectid = $scaleselect->getAttribute('id');
+        $scaleselectid = $this->generate_modgrade_subelement_id('modgrade_scale');
+        $scaleselect->updateAttributes(array('id' => $scaleselectid));
 
         // Maximum grade textbox.
         $langmaxgrade = get_string('modgrademaxgrade', 'grades');
         $maxgrade = @MoodleQuickForm::createElement('text', 'modgrade_point', $langmaxgrade, array());
         $maxgrade->setHiddenLabel = false;
-        $maxgrade->_generateId();
-        $maxgradeid = $maxgrade->getAttribute('id');
+        $maxgradeid = $this->generate_modgrade_subelement_id('modgrade_point');
+        $maxgrade->updateAttributes(array('id' => $maxgradeid));
 
         // Grade type select box.
         $gradetype = array(
@@ -107,7 +107,8 @@ class MoodleQuickForm_modgrade extends MoodleQuickForm_group{
         $langtype = get_string('modgradetype', 'grades');
         $typeselect = @MoodleQuickForm::createElement('select', 'modgrade_type', $langtype, $gradetype, $attributes, true);
         $typeselect->setHiddenLabel = false;
-        $typeselect->_generateId();
+        $typeselectid = $this->generate_modgrade_subelement_id('modgrade_type');
+        $typeselect->updateAttributes(array('id' => $typeselectid));
 
         // Add elements.
 
@@ -312,4 +313,17 @@ class MoodleQuickForm_modgrade extends MoodleQuickForm_group{
         return parent::onQuickFormEvent($event, $arg, $caller);
     }
 
+    /**
+     * Generates the id attribute for the subelement of the modgrade group.
+     *
+     * Uses algorithm similar to what {@link HTML_QuickForm_element::_generateId()}
+     * does but takes the name of the wrapping modgrade group into account.
+     *
+     * @param string $subname the name of the HTML_QuickForm_element in this modgrade group
+     * @return string
+     */
+    protected function generate_modgrade_subelement_id($subname) {
+        $gid = str_replace(array('[', ']'), array('_', ''), $this->getName());
+        return clean_param('id_'.$gid.'_'.$subname, PARAM_ALPHANUMEXT);
+    }
 }
diff --git a/lib/tests/formslib_test.php b/lib/tests/formslib_test.php
index 18e5edc5792..cfe314caba1 100644
--- a/lib/tests/formslib_test.php
+++ b/lib/tests/formslib_test.php
@@ -550,6 +550,33 @@ class core_formslib_testcase extends advanced_testcase {
         $data = $mform->get_data();
         $this->assertSame($expectedvalues, (array) $data);
     }
+
+    /**
+     * MDL-52873
+     */
+    public function test_multiple_modgrade_fields() {
+        $this->resetAfterTest(true);
+
+        $form = new formslib_multiple_modgrade_form();
+        ob_start();
+        $form->display();
+        $html = ob_get_clean();
+
+        $this->assertTag(array('id' => 'fgroup_id_grade1'), $html);
+        $this->assertTag(array('id' => 'id_grade1_modgrade_type'), $html);
+        $this->assertTag(array('id' => 'id_grade1_modgrade_point'), $html);
+        $this->assertTag(array('id' => 'id_grade1_modgrade_scale'), $html);
+
+        $this->assertTag(array('id' => 'fgroup_id_grade2'), $html);
+        $this->assertTag(array('id' => 'id_grade2_modgrade_type'), $html);
+        $this->assertTag(array('id' => 'id_grade2_modgrade_point'), $html);
+        $this->assertTag(array('id' => 'id_grade2_modgrade_scale'), $html);
+
+        $this->assertTag(array('id' => 'fgroup_id_grade_3'), $html);
+        $this->assertTag(array('id' => 'id_grade_3_modgrade_type'), $html);
+        $this->assertTag(array('id' => 'id_grade_3_modgrade_point'), $html);
+        $this->assertTag(array('id' => 'id_grade_3_modgrade_scale'), $html);
+    }
 }
 
 
@@ -822,3 +849,15 @@ class formslib_clean_value extends moodleform {
             'repeatnamedgroup[repeatnamedgroupel2]' => array('type' => PARAM_INT)), 'repeatablenamedgroup', 'add', 0);
     }
 }
+
+/**
+ * Used to test that modgrade fields get unique id attributes.
+ */
+class formslib_multiple_modgrade_form extends moodleform {
+    public function definition() {
+        $mform = $this->_form;
+        $mform->addElement('modgrade', 'grade1', 'Grade 1');
+        $mform->addElement('modgrade', 'grade2', 'Grade 2');
+        $mform->addElement('modgrade', 'grade[3]', 'Grade 3');
+    }
+}