The PSP lags behind sales of the Nintendo DS, which has shipped more than 40m units worldwide.

At a press conference in California, Sony also said it was working with Epic Games to optimise its Unreal graphics engine for use on the PlayStation 3.

Unreal Tournament 3, which uses the engine, will be available on PS3 in the Autumn. Seven developers, working on 20 more games, will use the optimised engine.

Sony also said it had secured exclusivity for much-hyped shooter Haze, at least for the "holiday season".

Nintendo conference

The firm also ended speculation that Metal Gear Solid 4, from veteran developer Hideo Kojima, would appear on the Xbox 360 as well as the PS3.

Sony said the game was "set for a simultaneous worldwide launch in early 2008 exclusively on PS3".

The press conference was also used to demo upcoming games, such as Killzone 2, and the latest iteration of the online portal, PlayStation Home.

Nintendo also held a press conference, at which it announced more games that it hopes will continue to broaden the gaming demographic.

Games announced included exercise title Wii fit, a new Mario Kart game for the popular console and the first Zelda game to appear on the Nintendo DS.

Nintendo of America president, Reggie Fils-Aime, said: "We're building on that foundation with games and accessories appealing to our current fan base and drawing in even more new players."