Briton Jenson Button will drive for BAR next year but is likely to join Williams in 2006 after Formula One's Contract Recognition Board (CRB) ended a tug-of-war between the teams on Wednesday.
The decision, announced three days after a hearing involving all parties in Milan, dashed the 24-year-old's hopes of an immediate return to the team with which he made his Grand Prix debut in 2000.
Button, with a career best third place in the championship secured, had agreed a two-year deal with BMW-powered Williams on advice from his management but BAR said their contract was watertight and still had a year to run.
Williams, preparing for this weekend's final Brazilian Grand Prix, said in a statement that they were disappointed but recognised the verdict and will not challenge it.
"Naturally we are disappointed with the CRB's conclusion. We took the view that Jenson was a driver worth challenging for, and based on strong legal advice, we have no regrets about making a bid for his services," said Frank Williams.
"The CRB accepted a significant number of the...team's arguments, but nevertheless found against us.
"We do not believe any further legal action in relation to this matter to be in the broader interests of the sport. In spite of the CRB's conclusion, which relates to 2005, we will maintain our close relationship with Jenson for 2006."
BUTTON DISAPPOINTED
The Swiss-based Contract Recognition Board is an independent body, backed by the governing International Automobile Federation (FIA), that resolves contract disputes in the notoriously litigious sport.
A driver can only enter the world championship if he has a recognised contract.
"Naturally, I am disappointed by the ruling, but as a matter of principle, I firmly believe in standing by the decision the CRB have reached today," said Button in the Williams statement.
"BAR have been extremely professional at the race track during this process, and I expect this to continue. I look forward to joining the BMW WilliamsF1 Team in the future."
Button, whose 10 podium finishes in 17 races have helped BAR become the revelation of the season, will stay and lead the team alongside confirmed Japanese driver Takuma Sato.
Williams wanted Button to partner Australian Mark Webber, signed from the soon to disappear Jaguar team, in what would have been an all-new lineup replacing Germany's Ralf Schumacher and Colombian Juan Pablo Montoya.
Schumacher is moving to Toyota and Montoya to McLaren.
The ruling will come as a blow to compatriots Anthony Davidson and David Coulthard who had hoped they might replace Button at BAR.
Williams may now opt for Brazilian Antonio Pizzonia, the test driver who stood in for the injured Schumacher in four races this season.
If they do, that would recreate Jaguar's 2003 line-up before that team dismissed Pizzonia for poor results.
Team boss Frank Williams has however spoken strongly of the Brazilian's potential.
Williams said they would make an announcement on their 2005 lineup "in due course".
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