Portal 2 might be released several days early on Steam, it has been revealed.

An Alternative Reality Game released by Valve has been pieced together to reveal a variety of messages, according to Kotaku.

One said that it needed "test subjects to move forward" and that a program named 'kick it' was released ahead of schedule.

Another provided a release date and time, Friday April 15 at 7am, which is assumed to be the new release date.

Finally, Gaijin Games' Bit.Trip Beat was updated with a new stage featuring Portal's GLaDOS, as well as a message stating that Portal 2 will be released early.

"Like the other 12 games in Valve's Potato Sack, it is rumored that playing Bit.Trip Beat between now and Portal 2's "scheduled" release date might increase the chances that Portal 2 is released early. Interesting idea..."

Additional reports have suggested that the title is already available for pre-loading, which is usually restricted until around 48 hours before a game's release.

It is unknown whether all Steam copies will be playable before release, or whether those who have purchased and played the Potato Sack bundle will gain access.

Last week it was revealed that Valve will announce downloadable content for Portal 2 after it is released.

Portal 2 will be available on Xbox 360, PlayStation 3, PC and Mac from April 19 in North America and April 21 in Europe.

> Read our recent hands-on preview with Portal 2

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