It's understood that BAR is seriously considering taking legal action following Jenson Button's shock defection to rivals WilliamsF1.

The news, which came last night, took almost everyone in F1 by surprise, no-one more so that Button's current boss David Richards.

In their 'who goes where' guides to 2005, virtually every specialist magazine, newspaper and website had regarded Button and BAR as a foregone conclusion, then came last night's bombshell.

It's understood that the move first became 'public' yesterday afternoon when Button's management company, Essentially Sport, issued a letter claiming that there was a question mark over "Honda's commitment". This comes two weeks after Honda revealed that it was renewing its contract with the British team, the same weekend that Button gave his strongest ever performance.

Consequently, a 'get out clause' supposedly allowed Button to tie up a new deal - rumoured to be for £10m a year - with the team that brought him into F1 in 2000.

Defending the 'deal', Frank Williams, talking to BBC Radio Five Live, said: "He has been under option to his present team BAR for some time. The option expired recently and Button's management called us to say that the option was no longer valid and would we be interested in his services - and of course I reacted as you might expect me to.

"I think they probably wanted to take up the option but whether they've executed or not will come out in due course," he added.

"To be treated like this is dumbfounding," David Richards told The Times. "We are all utterly shaken. 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.

"I cannot believe that Jenson wants to leave," he continued. "There is a hidden agenda here somewhere and we are determined this is not going to happen. Somebody has been looking for a get-out clause because there is absolutely no doubt Honda are committed. Honda only announced they were renewing and extending their contract with us before the race in Germany, so why should there be a doubt?

"We believe we can make Jenson Button a world champion and we, with Honda, have proved this year that we are going in the right direction and we thought we were all going together," he added. "Perhaps Jenson has been led in another direction - down the garden path. I can only hope that I can talk to him and make sense of what is going on."