The Formula One future of driver Jenson Button was still in doubt after the BAR team handed the Briton, who has signed a contract to join Williams next year, an ultimatum on Friday.

BAR have called Button to a face-to-face meeting with the team by Monday evening or he will face a legal fight over his move to Williams in 2005.

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The 24-year-old Briton has ignored calls from BAR since Williams announced late on Thursday that they had signed him to race alongside Australian Mark Webber.

BAR had a long-term option on Button, who is out of contract at the end of the season, and BAR team principal David Richards insists they had taken that up.

"We have been trying to talk to Jenson for days," said Richards. "He has a clear duty to all of those who've worked with him for the last two years to explain to them what is going on.

"We have complete confidence in the strength of our contractual rights and in the option that we have exercised for 2005," Richards added. "If people make commitments they should keep them.

"If we do not have a meeting with Jenson by Monday night to sort this out, the processes to enforce our rights under the contract will commence without further notice."

The team said in a statement that the option had been approved at a BAR board meeting on July 20, adding that Button and his management team were informed in writing and had acknowledged the decision before the July 31 deadline.

Notice of exercise of the option had also been lodged with the Formula One Contract Recognition Board in Switzerland.

"I will use every means available to make sure all the requirements of our contract are met," Richards added.

WILLIAMS CONFIDENCE

Williams, whose team has had a contract with Button since he made his Grand Prix debut with them in 2000, said the Briton's management notified him of the opportunity before the German Grand Prix on July 25.

The move is understood to have been made possible because of questions in the solidity of BAR's relationship with engine partners Honda, who in July renewed their contract to the end of 2007.

In a conference call with reporters on Friday, Williams chief Frank Williams, who said he initially wanted discussions to remain private, insisted: "The first clear and unequivocal point is that we hold a contract with Jenson.

"It appears that BAR contest this. However, we are of the firm opinion, on strong legal advice, that BAR lost the opportunity to obtain Jenson and I have no doubt he will be with the Williams-BMW F1 team for 2005."

BUTTON DEFIANT

Button told BBC Radio Five Live he would be with Williams in 2005.

"I am going to be driving for Williams next season, I'm not a legal expert but from the information I have I am able to race for Williams next year," he said.

"BAR have been a really great team to work with and I've enjoyed every moment with them. The atmosphere will be a bit different now but these things do happen and Formula One is not just about this season.

"My main aim in Formula One is to be world champion and I want to challenge Michael Schumacher for the next few years and to do that Williams is the best team."

Richards rescued Button from a difficult period at Benetton and nurtured him into a potential champion.

Button scored his first podium in this year's Malaysian Grand Prix and lies third in the world championship after claiming a further six podiums, including second place in the last race in Germany.