Rival team Williams said last Thursday they had signed the British driver on a two-year deal due to start at the end of this year after BAR had failed to exercise an option on his services beyond the end of his current contract.

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But in a statement BAR said: "The FIA Contract Recognition Board (CRB) acknowledged, in writing, that, as of today, the BAR contract with Jenson Button is the only valid contract registered for the 2005 season."

The CRB was established in 1992 to regulate driver contracts and resolve disputes where a driver enters into a new contract when an established and registered one is already in existence.

BAR say this is the case with Button and as Williams have to operate under the rules of the Concorde agreement that governs the sport they should have registered their contract with Button when it was signed.

"A driver's super licence, his right to drive in Formula One, is only valid if his contract has been lodged with and registered by the CRB," the BAR statement said.

"Any new driver contract (must be) registered 'forthwith', in other words immediately, and without delay, in order to establish its validity.

"BAR has willingly submitted itself to this process, which is final and binding on all parties."

BAR said they took the decision to take up the option on Button's contract on July 20 and informed the driver's management team. They also lodged the decision with the CRB.

Button, who made his grand prix debut with Williams in 2000, met BAR team chief David Richards on Sunday to explain the controversial move.

"David is now in no doubt of my intention to return to BMW-Williams in 2005. My contractual position allows for this," the 24-year-old said in a statement after that meeting.

The dispute seems destined to go through the courts.

Richards has said he will not drop Button, who is third in the drivers' championship, for this weekend's Hungarian Grand Prix.