Valve has announced that its four year-old team-based shooter, Team Fortress 2, has become a free-to-play title supported entirely via microtransactions.

In-game clothing, weapon and customisation options, already available via Steam's Wallet purchase system, will now fund the game in its entirety.

The announcement follows Valve allowing third-party free-to-play games to be featured on its Steam download service earlier this month, launching with Sega's Spiral Knights, Perfect World's Forsaken Worlds, Atari's Champions Online: Free for All, Hi-Rez Studios' Global Agenda: Free Agent, and NHN USA's Alliance of Valiant Arms.

Team Fortress 2 is the first Valve made game to follow suit. There will be no ads in game and a detailed FAQ on the game's official website explains that by nearly all purchasable items will also be available via random in-game drops too, meaning that it's possible to unlock almost everything for free with patience.

Players downloading the game now will be registered as having a 'free account'. Anyone who previously paid for the game, or who uses their Steam Wallet to make a purchase from the in-game Mann Co. Store will be upgraded automatically to a 'premium account'.

"Premium accounts have a few extra features, including access to rare and cosmetic items through random item drops, the ability to store more items in your backpack, and more powerful trading and crafting abilities," reads the FAQ. "Otherwise, the gameplay experience will be identical for both accounts.

"The entire game can be played without making a purchase. All game modes, classes, and maps are available. Nearly every weapon is available through achievements, drops, or crafting."

When in-game purchases were launched for Team Fortress 2 last year, five modders won a competition to have their items featured in the Mann Co. Store, taking 25 per cent of all revenues which they raised. Within two weeks, the earnings for the most successful items were around $140,000, netting nearly $50,000 for their creators.