BAR team principal David Richards promised to challenge Jenson Button's move to Williams before gathering his team for a crisis meeting at their Milton Keynes headquarters on Friday.
Richards was shaken by the news late on Thursday that the in-form 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 on Friday, 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 rival team chief Sir Frank Williams was offered the driver on a two-year deal and suggested "they probably didn't execute it (the option) properly".
BUTTON'S DEBUT
Button has been impressive this season and is targeting second place in the drivers' championship after closing the gap on Ferrari's Rubens Barrichello in July's German Grand Prix.
Williams gave Button his Grand Prix debut in 2000 before ditching him for Colombian Juan Pablo Montoya but it is understood the British driver has decided to return because he is not confident about the future of his current team.
Richards told BBC Radio on Friday: "About 36 hours ago we had a letter suggesting that on a very slim technicality the option (on Button) wasn't viable based upon Honda's commitment to us.
"Obviously, this is something that is quite ridiculous and we will challenge it. You can't go through life taking advantage of legal technicalities -- you have to look at the essence of an agreement."
At the last Grand Prix at Hockenheim in Germany, Honda announced the continuation of their partnership with BAR until the end of 2007 and promised to become more involved in chassis development as well as engines.
They also confirmed there is a get-out clause in their deal which will allow them to leave the sport if proposed new rule changes remove its technical challenge but Richards insisted the Japanese company are fully committed.
The move is a massive blow to BAR, who had manoeuvred Button into a position as their effective figurehead and expected him to lead them to the world championship in the next two years.
A BAR team spokesman said: "The first priority was to gather the team together and discuss the situation and it was a long meeting. This has suddenly become a very busy period for us."