Source: forum.joomla.org

I haven't tested it yet (as you can see) but it's the only solution I found that doesn't include installation of an extra Component:

1. Connect with your favorite FTP program to your Server, find “components/com_content/router.php“, download and make a backup of the file ... then edit it.
   
   
2. Find and Replace:
   $advanced = $params->get('sef_advanced_link', 0);
   With this:
   $advanced = $params->get('sef_advanced_link', 1);
   
   There are two places need to do this change, one near the top, one in the middle
   
   
3. You want to edit your router.php file:
   - From: ->where($db->quoteName(‘alias’) . ‘ = ‘ . $db->quote($db->quote($segment)));
   - To: ->where($db->quoteName(‘alias’) . ‘ = ‘ . $db->quote($segment));
   
   
4. Save, reupload the file and test it.

Update: Tested and working perfectly. Be sure to search for the line in step 3 thoroughly in case the copy paste doesn't work.