Jenson Button has told his BAR team boss David Richards that there is no going back on his switch to Williams.
Button told Richards he was moving to Williams because they offer him a better chance of winning the title.
"I have made my decision, my contractual position allows this and I clarified I am not moving for money," Button said in a statement.
And Button criticised Richards for handling his decision in a more "acrimonious" manner than necessary.
"Drivers are changing teams up and down the paddock at the moment - an accepted and fundamental element of our sport," Button said.
"And I wish that my return to Williams would have been less acrimonious in light of the contribution I have made to BAR this season.
"I have worked hard and contributed seven podiums and 61 of the team's 76 points this season.
"I do not feel it was constructive for BAR to leak details of these discussions."
Button said he thought Richards should have been more understanding of his desire to move to one of F1's most successful teams.
"My motivation is straightforward determination to win a World Championship, determination which I am sure he as a competitive person well understands," Button said.
"Naturally, the main element of disagreement is whether these ambitions would be better served by BMW Williams or BAR."
Richards said on Friday his team would take legal action over the move if Button did not explain his actions.
The Briton, who is set to race for BAR at next Sunday's Hungarian GP, is focused on maintaining the run of form that has seen him become the leading non-Ferrari driver.
And while both BAR and Williams claim their contracts are valid, Button said he is confident he will be at the wheel of a Williams car next season.