Wurz joined Williams at the start of the season

Wurz, who joined the team as a third driver in January, will line up with Nico Rosberg for the 2007 campaign.

Webber, 29, has only finished four races in 2006 and will leave Williams at the end of the season.

The Australian is managed by Renault team boss Flavio Briatore and could now replace world champion Fernando Alonso, who joins McLaren next year.

Webber's current Williams deal included an option for the team to retain him for 2007, but that expired last weekend with no agreement.

"This year has been tough but we are seeing the beginnings of a performance upswing which we are determined to continue," said boss Frank Williams.

Alex will be a great racing driver for us as Team boss Frank Williams

"Together with our recent announcement regarding our Toyota engine partnership, we are starting to put in place some of the cornerstones of a competitive team for the rest of this season and into the future."

Williams, who have not won a world title since 1997, have struggled this season and are currently in eighth position in the constructor standings with just 10 points.

Webber joined Williams in 2005 after a stint with the then Jaguar team but with an under-performing car, he has only been able to make the podium once.

"I must commend Mark for his contribution to the team over the past two seasons as he is a credit to himself and Williams," Williams added.

"He has never let up in his pursuit of competitive success and I know Mark will give his all until the end of the season. We wish him well in his future arrangements."

Wurz, a former McLaren test driver, steps up after a season as the driver of Williams' third car during Friday practice.

Now my role takes on additional responsibility and I am really looking forward to this Alexander Wurz

"Alex brings us important technical continuity and understanding of the car and as well as being an excellent communicator with our engineers, he is ultra-competitive during the Friday sessions," said Williams.

"Alex will be a great racing driver for us as he is one of the few men I have seen in a wheel-banging confrontation with Michael Schumacher and come out on top."

Wurz, 31, joined McLaren as a test driver in 2001 from Benetton.

In 2005 he replaced the injured Juan Pablo Montoya for the San Marino grand prix and finished fourth but was elevated to third after Jenson Button was stripped of his podium place.

"My natural habitat is racing, so I couldn't be happier to have received Williams' vote of confidence for this job next season," said Wurz.

"Of course I know the team well having been a part of the race programme at every grand prix this season, but now my role takes on additional responsibility and I am really looking forward to this."