Britain's Jenson Button has announced he will race for Williams next season after signing a two-year deal.
BAR reacted by claiming they have taken up an option for the 24-year-old to remain as one of their drivers.
But Button believes a return to his old team provides the "best platform" for him to become world champion.
"I'm very pleased that I had the option to rejoin the BMW Williams F1 Team," he said. "But for the meantime, the 2004 season has my full focus."
"I have every confidence that the massive investment in resources and the depth of talent at Williams F1 and BMW provides the best platform for my future ambitions to be a world champion."
Speaking before BAR issued their statement disputing Button's move, Williams boss Sir Frank Williams told BBC Radio Five Live that he believed BAR's option had expired.
"He has been under option to his present team BAR for some time," said Williams.
"The option expired recently and Button's management called us to say that the option was no longer valid and would we be interested in his services - and of course I reacted as you might expect me to.
"I think they (BAR) probably wanted to take up the option but whether they've executed or not will come out in due course."
Asked whether Button can win a world title within two years, Williams was unequivocal.
"If we give him a good enough car, most certainly yes," Williams said. "He's on a strong upward path, every drive he improves.
"We've always believed an outstanding measure of a great world champion driver is the smoothness with which he drives and the infrequency of his accidents."
If the move does go ahead Button will complete Williams' 2005 driver line-up after the recent signing of Australian Mark Webber, with current drivers Juan Pablo Montoya and Ralf Schumacher leaving the team.
The Briton has been enjoying a superb season with the much-improved BAR team and currently lies third in the drivers' championship with 61 points from 12 races.
And Williams believes Button was attracted to his former team because of recent investment in the outfit.
"He started with us in 2000 and we've been in very close contact all of the time since.
"Maybe he just harboured a wish (to return) and recognised we had spent tens of millions of pounds reinvesting in the company technically speaking.
"Maybe he recognises we have a greater opportunity of providing him a world championship car than his present team."
Button became Britain's youngest ever F1 driver and the fifth youngest ever when he raced for Williams at the Australian Grand Prix in 2000.