Austrian Alexander Wurz will be test and reserve driver for Williams this season, the team confirmed today.
Wurz, 31, has been a McLaren tester for the last five seasons after racing for Benetton from 1997 to 2000.
"I am delighted with the agreement we have reached with Alex," said Frank Williams in a statement.
"Everyone at Williams believes he will bring significant improvements to the team's performance, on grand prix Fridays in particular, and to our testing programme in general."
Williams finished last year in fifth place overall and are now entitled to run a third car in Friday practice, unlike runners-up McLaren.
Wurz will deputise for Australian Mark Webber or Germany's Nico Rosberg should either be unable to race and the Austrian said he had always wanted to drive for Williams.
"I feel full of energy to be part of Williams' 2006 season attack and look forward to working closely with Mark and Nico," he said.
"I believe that together we will achieve great results."
Williams are powered by Cosworth engines this year after splitting from BMW, who have bought Swiss-based Sauber. Wurz had been linked to a race seat with that team before they confirmed Canadian Jacques Villeneuve.
The Austrian's appointment dashed the hopes of Indian Narain Karthikeyan, who had tested twice for Williams before the Christmas holiday break after racing for Jordan last year.