BAR have won their fight with Williams for Jenson Button to drive for them next season.
A contractual dispute about the driver's future was decided in BAR's favour following a ruling by Formula One's Contract Recognition Board (CRB).
Button had signed for Williams citing a loophole in his 2005 BAR contract as a way to return to the team.
He said: "Naturally I am disappointed but as a matter of principle I firmly believe in standing by the decision."
But the 24-year-old vowed to return to Williams in the future.
"BAR have been extremely professional at the race track during this process, and I expect this to continue," he said.
"I look forward to joining the Williams team in the future."
Williams chief Frank Williams also said he would accept the decision, but suggested he will attempt to sign Button for 2006.
"Naturally we are disappointed with the CRB's conclusion," Williams said.
"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.
"The CRB accepted a significant number of the BMW WilliamsF1 Team's arguments, but nevertheless found
"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."
Legal presentations from both teams were heard in Milan on Saturday before the verdict was delivered.
Williams announced the decision from Interlagos where they are preparing for the Brazilian Grand Prix.
The dispute began in August when Williams claimed they had a prior option on Button.
They believed they were able to exercise the option because Honda's commitment to supply engines to BAR was not definitive in a new contract drawn up this summer.
BAR argued that Honda's intent was clear.
BAR boss David Richards, who always insisted he was the only man to have a genuine contract with Button, was overjoyed at the he CRB's decision.
"I'm delighted to confirm that we just received the official
notification that the CRB have ruled in our favour," he said.
Button has inspired BAR to second spot in the Constructors' Championship behind Ferrari while he himself lies in third place in the drivers' standings.
The 24-year-old Somerset-born driver has started to realise his potential this season with some impressive displays.
Brazilian Antonio Pizzonia is now likely to step in at Williams after impressing as a recent stand-in for Ralf Schumacher.