Valve's confirmed to GameSpot that Portal has sold nearly four million copies worldwide, and that figure excludes Steam sales.

Portal began life as Narbacular Drop, a student project at the DigiPen Institute of Technology. The game won the Independent Game Festival's Student Showcase Award in 2006 and its creators were soon snapped up by Valve.

An adapted version of Narbacular Drop became Portal, released in 2007 as part of Valve's The Orange Box.

Portal 2 was released in the early hours of Tuesday morning via Steam, while console gamers must wait until tomorrow when the game hits stores.