Portal 2's first DLC pack has been released by Valve.

The 1.37GB download is now available on Xbox 360, PlayStation 3, PC and Mac for free.

To celebrate, the Steam price of Portal 2 has been lowered to £9.99 on PC and Mac for a limited time.

The 'Peer Review' DLC adds a new test track for co-operative mode that continues the story of Atlas, P-Body and GlaDOS.

Elsewhere, it adds Challenge modes to the single and multiplayer modes, as well as leaderboards that allow users to compare portal placement and times.

Valve first confirmed new downloadable content for Portal 2 in April, before announcing that it would release in September.

> Read our review of Portal 2

Watch gameplay footage of how the Challenge Mode works in Portal 2's campaign below:

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