Jenson Button left F1's Brackley based team to prove to his critics that he is a worthy world champion, according to Ross Brawn.

Jenson Button left F1's Brackley based team to prove to his critics that he is a worthy world champion, according to Ross Brawn.

When Button, 30, announced as the new champion that he was leaving Brawn to seek a new challenge in 2010 alongside Lewis Hamilton at McLaren, that explanation was not universally believed.

At the time the Briton apparently made his decision, Nico Rosberg was already under contract for 2010 and rumours were building that Michael Schumacher was set to launch a full return to formula one to drive Mercedes' new works car.

Paddock rumours are still rife that Button's "new challenge" reason was a cover-story for Brawn effectively ousting the new reigning world champion.

But Brawn said on Monday: "I was disappointed Jenson left.

"He felt people were of the opinion he only won the world championship because he had the best car. So he left us to show them he could do it again elsewhere," he told The Sun.

"He also wanted to prove himself at McLaren against the guy who is probably the fastest and most naturally talented in motor racing -- Lewis Hamilton," continued Brawn.

"He's set himself a massive challenge and he is doing very well so far."

Brawn insists that the split did not affect his off-track friendship with Button, who is currently leading the world championship after winning two of the opening four races.

"We are still friends. We were on the same flight back from Malaysia and we had a good chat.

"Jenson was a fabulous member of our team last year but now he is the history and we have to beat him," added Brawn.

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