Sessions working differently with and without WWW in url

I've used the Codeigniter sessions library for user authentication.

Everything was working fine,
but i now noticed that sessions created by a page on http://mysite.com are not being recognized by pages on http://www.mysite.com

Seems like this problems has been present from the beginning. I could just notice it now.

Can someone who had the same problem and had solved it, please help me?