Australian Mark Webber will leave Jaguar to drive for Williams in 2005.
Webber will finally get a chance to drive in a top team
Team owner Frank Williams said: "Mark is a highly talented driver, but his tenacity, determination and motivation are also qualities we prize.
"We have traditionally prospered with
drivers who have a down-to-earth, 'let's get on with it' attitude. Mark is certainly in that mould."
Webber said: "This is my career's most significant milestone and I have great expectations for what we can achieve."
Williams' securing of Webber signature marks the end of a long pursuit - he has always been at the top of their list for 2005.
Ferrari: M Schumacher, Barrichello
Renault: Alonso, Fisichella
BAR: Button, Sato*
Williams: Webber, ?
McLaren: Raikkonen, Montoya
Toyota: R Schumacher, Trulli*
* = to be confirmed
The Australian said: "Besides having an amazing history of success, the team has a reputation for a no-nonsense and straightforward appraoch to racing, which suits me just fine."
Williams needed to replace both drivers for 2005 after Juan Pablo Montoya's move to McLaren and Ralf Schumacher's decision to leave for Toyota.
But the team are still searching for a second driver after losing their other favoured option, Giancarlo Fisichella, to Renault.
It is rumoured that Renault team boss Flavio Briatore, who manages Webber, was insistent that he wanted one of the two drivers.
A deal was brokered that saw Williams free up Fisichella from Sauber - whose contract with the Italian only allowed him to leave to join Ferrari, Williams or McLaren - and then released him to Renault in return for Webber.
Jordan's Nick Heidfeld, McLaren star David Coulthard, BAR test driver Anthony Davidson and former world champions Jacques Villeneuve and Mika Hakkinen are all options for the second Williams seat.
Jaguar team boss David Pitchforth paid tribute to the job Webber had done for the team and confirmed he would stay for the rest of the season.
Pitchforth added: "Mark has now reached a stage where he needs to continue this progress and pursue race wins. We understand his decision and respect it."