Germany's Nick Heidfeld has won the race to partner Australian Mark Webber at the BMW Williams team this season.
Heidfeld forced himself into the frame for the Williams drive
Heidfeld has impressed the team sufficiently in recent tests to edge out Brazilian Antonio Pizzonia, Williams' long-time reserve driver.
"I really wanted this job. Driving for Williams is the greatest opportunity of my career. My goal is to win the title in the future," Heidfeld said.
Pizzonia, who will continue as test driver, said he was "disappointed".
The team waited until the launch of their 2005 car to reveal the driver. Team owner Frank Williams said the decision had been made "at the last minute".
"It wasn't an easy decision for us to make," Williams added.
"There was little to choose between the two of them, and we were in the fortunate position to be able to choose between two drivers who would be a credit to any Formula One team.
"Ultimately, however, it is Nick who has got our vote, but I am delighted that we have strength in depth with Antonio taking up the position of official test and reserve driver."
Heidfeld added: "I will do everything I can to show that I deserve this place and I'm counting the days to the first Grand Prix.
"'Racing' against Antonio was a lot of fun. He's a great team- mate. I respect him and I would also have taken on the job of test driver."
The second Williams drive represents Heidfeld's big chance to make a name for himself after several years marking time in F1.
He was part of McLaren-Mercedes' driver development programme but after several years as their test driver the team decided not to promote him to a race drive.
Instead he made his race debut with the now-defunct Prost team in 2000.
Heidfeld moved to Sauber in 2001 and was further snubbed by McLaren that year when they decided to turn instead to his team-mate Kimi Raikkonen when double world champion Mika Hakkinen retired.
Heidfeld moved to Jordan in 2004 and impressed with several forceful drives for the struggling outfit.
Williams decided to assess his potential after Jenson Button's switch from BAR was blocked by Formula One's contract recognition board.
Team boss Frank Williams admitted earlier this month that Pizzonia had initially been first choice but engine partner BMW suggested the team take a look at Heidfeld.
"I did all I could and now it is just a question of waiting," said Heidfeld last week.
Williams conceded that the appointment of Pizzonia might not go down well with Webber.
The Australian dominated Pizzonia when they were team-mates at Jaguar, and was angered by recent claims from the Brazilian that Webber got preferential treatment.
The team took a similar late decision in 2000, when Button was chosen over Brazilian test driver Bruno Junqueira.
Button was told minutes before the launch in Barcelona that he had got the nod, with Williams later admitting the decision not to go with Junqueira had been taken over breakfast.