Richards says he has not spoken to Button about his decision

Button has signed to move to Williams for 2005, and both teams insist theirs is the valid contract.

BAR team boss David Richards said: "We have been trying to talk to Jenson for days. He has a clear duty to explain what is going on.

"We have complete confidence in the strength of our contractual rights."

Williams firm on Button

Earlier on Friday, rival team owner Frank Williams insisted "on strong legal advice" that his team had prior call on Button.

The row is likely to go to F1's contracts recognitions board if it cannot be solved by talks between the two teams before that.

Richards said BAR's option on Button had been approved at a team board meeting on 20 July.

Analysis: Why Button wants to move to Williams

"Jenson and his management team were informed of this in writing well before the end of the option period on 31 July, a fact acknowledged by Essentially Sport (Button's management).

"Notice of exercise of the option had also been lodged with F1's contract recognition board in Switzerland.

"If people make commitments they should keep them - and I will use every means available to me to make sure that all the requirements of our contract are met.

Going back to Williams shows only loyalty to himself, and will only reduce the breadth of appeal the sport currently has From Tony Andrews, Coalville Join the debate

"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."

A BAR spokeswoman said it was premature to comment on whether that meant Button could be dropped from the Hungarian Grand Prix on 15 August.

"David just wants to sit down with Jenson and find out what's going on in his mind before taking things any further," she told BBC Sport.

It is believed that Button's management have taken advantage of clauses in engine supplier Honda's new contract with BAR.

The contract is understood to reflect Honda's concerns about the future of the team in the context of the possibility of a global tobacco advertising ban being introduced within the next couple of years.

BAR is owned and almost entirely funded by tobacco giant BAT.

Richards insisted earlier on Friday that "it would be unfair to criticise Jenson until we can personally find out what is going on" but added that "400 people at the factory want an explanation".

Driver profile: Jenson Button

Frank Williams said on Friday: "Whatever is said about contractual details, one fact is not contestable. Jenson wishes and expects to drive for the BMW Williams F1 team in 2005 and 2006 and has made this intention clear."