Engineers Nick Alcock and Jason Somerville are not seen as direct replacements for Willis, and their new job titles have yet to be finalised.

They will be supported in the short-term by Gavin Fisher, the team's chief designer. He will take on temporary extra responsibilities.

The situation is reminiscent of that in 1997 when Adrian Newey left Williams to become McLaren's technical director and Fisher and Willis were promoted to replace him from within the team.

That decision was one of the contributing factors to Williams spending two relatively uncompetitive seasons in 1998 and 1999.

Willis does not move to BAR until March 2002, by which time the design of the new Williams car will be finalised.

"To avoid confusion, I will be called engineering director," he said.

"I have been conscious for some time that the job I've been trying to do here is too much for one person.

"At the same time, we have been missing the senior technical experience that can only come from teams at the top of the championship.

"Geoffrey fits the bill perfectly and I'm pleased he has agreed to join us."

Williams finished the 2001 season third overall with four race wins, while BAR were sixth and have yet to win a race.

Willis said the opportunity was one he could not turn down.

BAR had "the resources in place to achieve its ultimate ambition of winning the championship," he said.