Nico Rosberg © Gallo Images

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The 24-year-old has spent the last four seasons at Williams, with second place in Singapore last year his best result in 70 starts.

Mercedes has been the engine supplier to McLaren, who will have an all-British line-up next year with Lewis Hamilton and Jenson Button, since 1995, but has now taken over British-based champions Brawn GP.

Brawn, which emerged from the remains of the Honda team, won both titles in its debut 2009 season with Button and Brazilian Rubens Barrichello, who has taken Rosberg's place at Williams.

"It's quite a special thing," Rosberg told reporters. "It's about time that a German was in a Silver Arrow again."

Rosberg, whose signing was an open secret in Formula One circles, expects to win races next year.

"I am going to have a great car and I really believe that I am going to be able to take them (Hamilton and Button) on," he said, also naming Red Bull and Ferrari as leading rivals.

Team principal Ross Brawn, who worked with Keke, described Nico as a great talent and "a driver who will be able to make a valuable contribution to our team right from the outset."

Mercedes has yet to name its second driver, amid persistent rumours that former champion Michael Schumacher could come out of retirement at the age of 41 to fill the vacancy left by Button. Asked to rule out the possibility, Mercedes motorsport vice-president Norbert Haug and Brawn chief executive Nick Fry both failed to do so.

"We are here today to talk about Nico and I think we should stick to that subject," said Fry.

"I fully understand that these speculations are ongoing as long as we did not announce who is driving the other car, but that's what we have to live with at the moment," added Haug.

Schumacher's spokeswoman said at the weekend that she considered a return "highly unlikely at the moment".

Rosberg said he did not mind who Mercedes signed, providing he was a team player, but would not comment on Schumacher.

Finland's 2007 champion Kimi Raikkonen, who has announced plans to take a year out after leaving Ferrari, could be another option while Germany's Nick Heidfeld is also available.

Fry said the approach to Rosberg had been made during the season and before Mercedes expressed an interest in buying the team.

"Nico was due to join Brawn under any circumstance," he said. "We are extremely pleased that the circumstances have worked out as they have done but the negotiation was done principally with the Brawn team and myself and Ross."

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