BAR team principal David Richards has vowed to challenge Jenson Button's move to Williams but rival team chief Frank Williams said the deal is secure.
Richards was shaken by the news late yesterday that the British driver, who he rescued from a difficult period at Benetton and nurtured into a potential champion, has decided to defect next season.
The BAR-Honda team failed to contact Button last night and Richards, who will make a further statement later today, told The Times: "To be treated like this is dumbfounding and I cannot believe that Jenson wants to leave.
"I have got 400 people in this factory who have worked their socks off night and day and then the guy who gets all the credit and drinks the champagne turns around and ditches them."
BAR had a long-term option on Button and had been expected to take it up but Williams was offered the driver by Button's management on a two-year deal and suggested "they (BAR) probably didn't execute it (the option) properly".
In a conference call with journalists today, Williams 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.
"Whatever is said about contractual details, one fact is not contestable - Jenson wishes and expects to drive for Williams-BMW in 2005 and 2006 and has made this intention clear."
Williams said the deal was initiated a few days before the German Grand Prix on July 25th, when he was contacted by Button's management and told of the opportunity.
He said that Williams have held a contract with the British driver ever since he made his Grand Prix debut with the team in 2000 and had always anticipated he would return.
"We wished to open a dialogue privately with BAR to run through all the contractual issues, however BAR pre-empted us, went to the press, and we have been obliged to make our position clear," said Williams.
He said he had contacted Richards just after the BAR chief would have received notification of the deal and suggested the two parties discuss the situation privately.
But he admitted: "We assumed (signing Button) was out of the question but this is an opportunity I have experienced in the past and being a racing team we went for it," he said.