diff --git a/cache/classes/loaders.php b/cache/classes/loaders.php
index 691d2fb540d..6b1337f050d 100644
--- a/cache/classes/loaders.php
+++ b/cache/classes/loaders.php
@@ -1712,6 +1712,7 @@ class cache_session extends cache {
     public function __construct(cache_definition $definition, cache_store $store, $loader = null) {
         // First up copy the loadeduserid to the current user id.
         $this->currentuserid = self::$loadeduserid;
+        $this->set_session_id();
         parent::__construct($definition, $store, $loader);
 
         // This will trigger check tracked user. If this gets removed a call to that will need to be added here in its place.
@@ -1771,8 +1772,6 @@ class cache_session extends cache {
                 // Purge the data we have for the old user.
                 // This way we don't bloat the session.
                 $this->purge();
-                // Update the session id just in case!
-                $this->set_session_id();
             }
             self::$loadeduserid = $new;
             $this->currentuserid = $new;
@@ -1780,8 +1779,6 @@ class cache_session extends cache {
             // The current user matches the loaded user but not the user last used by this cache.
             $this->purge_current_user();
             $this->currentuserid = $new;
-            // Update the session id just in case!
-            $this->set_session_id();
         }
     }
 
diff --git a/cache/tests/cache_test.php b/cache/tests/cache_test.php
index 6aa268fc12f..9c0f1d93475 100644
--- a/cache/tests/cache_test.php
+++ b/cache/tests/cache_test.php
@@ -2327,4 +2327,51 @@ class core_cache_testcase extends advanced_testcase {
         $this->assertEquals('test data 2', $cache->get('testkey1'));
     }
 
+    /**
+     * Test that values set in different sessions are stored with different key prefixes.
+     */
+    public function test_session_distinct_storage_key() {
+        $this->resetAfterTest();
+
+        // Prepare a dummy session cache configuration.
+        $config = cache_config_testing::instance();
+        $config->phpunit_add_definition('phpunit/test_session_distinct_storage_key', array(
+            'mode' => cache_store::MODE_SESSION,
+            'component' => 'phpunit',
+            'area' => 'test_session_distinct_storage_key'
+        ));
+
+        // First anonymous user's session cache.
+        cache_phpunit_session::phpunit_mockup_session_id('foo');
+        $this->setUser(0);
+        $cache1 = cache::make('phpunit', 'test_session_distinct_storage_key');
+
+        // Reset cache instances to emulate a new request.
+        cache_factory::instance()->reset_cache_instances();
+
+        // Another anonymous user's session cache.
+        cache_phpunit_session::phpunit_mockup_session_id('bar');
+        $this->setUser(0);
+        $cache2 = cache::make('phpunit', 'test_session_distinct_storage_key');
+
+        cache_factory::instance()->reset_cache_instances();
+
+        // Guest user's session cache.
+        cache_phpunit_session::phpunit_mockup_session_id('baz');
+        $this->setGuestUser();
+        $cache3 = cache::make('phpunit', 'test_session_distinct_storage_key');
+
+        cache_factory::instance()->reset_cache_instances();
+
+        // Same guest user's session cache but in another browser window.
+        cache_phpunit_session::phpunit_mockup_session_id('baz');
+        $this->setGuestUser();
+        $cache4 = cache::make('phpunit', 'test_session_distinct_storage_key');
+
+        // Assert that different PHP session implies different key prefix for storing values.
+        $this->assertNotEquals($cache1->phpunit_get_key_prefix(), $cache2->phpunit_get_key_prefix());
+
+        // Assert that same PHP session implies same key prefix for storing values.
+        $this->assertEquals($cache3->phpunit_get_key_prefix(), $cache4->phpunit_get_key_prefix());
+    }
 }
diff --git a/cache/tests/fixtures/lib.php b/cache/tests/fixtures/lib.php
index 6c42c39aff9..bee66090d1a 100644
--- a/cache/tests/fixtures/lib.php
+++ b/cache/tests/fixtures/lib.php
@@ -465,6 +465,9 @@ class cache_phpunit_application extends cache_application {
  */
 class cache_phpunit_session extends cache_session {
 
+    /** @var Static member used for emulating the behaviour of session_id() during the tests. */
+    protected static $sessionidmockup = 'phpunitmockupsessionid';
+
     /**
      * Returns the class of the store immediately associated with this cache.
      * @return string
@@ -480,6 +483,31 @@ class cache_phpunit_session extends cache_session {
     public function phpunit_get_store_implements() {
         return class_implements($this->get_store());
     }
+
+    /**
+     * Provide access to the {@link cache_session::get_key_prefix()} method.
+     *
+     * @return string
+     */
+    public function phpunit_get_key_prefix() {
+        return $this->get_key_prefix();
+    }
+
+    /**
+     * Allows to inject the session identifier.
+     *
+     * @param string $sessionid
+     */
+    public static function phpunit_mockup_session_id($sessionid) {
+        static::$sessionidmockup = $sessionid;
+    }
+
+    /**
+     * Override the parent behaviour so that it does not need the actual session_id() call.
+     */
+    protected function set_session_id() {
+        $this->sessionid = static::$sessionidmockup;
+    }
 }
 
 /**