Using Sub-Directories for Default_controller

Hi Everyone, Apologies if this has been asked and answered else where but I couldn't find anything.

Basically, when setting the default controller to:

PHP Code:

$route['default_controller'] = 'welcome'; 


everything works as expected but as soon as I set the default controller to a file within a sub directory it doesn't work as per below:

PHP Code:

$route['default_controller'] = 'admin/dashboard'; 


The file has an index() function therefore accessing the method/function is not a problem and I've also tried the following just in case:

PHP Code:

$route['default_controller'] = 'admin/dashboard/index'; 


It definitely worked in codeigniter 2.4 but from a little bit of reading online it appears this has been changed in codeigniter 3, is there anything I can do to rectify this?

Thanks
Chris