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.
"Intelligent" Button Winning Battle With Hamilton
IT IS Jenson Button's "really intelligent approach" that is paying off so far in 2010, according to his former boss Ross Brawn.
After two wins on damp tracks so far in 2010, the reigning world champion is currently leading the world championship.
But throughout the winter, most experts predicted that Button, 30, would be blown away at least initially by Lewis Hamilton.
His friend David Coulthard wrote in his latest Telegraph column: "I told you so."
Brawn, who replaced Button with Michael Schumacher this year but insists the Briton switched to McLaren for the new challenge, told Reuters that F1 is again seeing the "natural Jenson" who won many races early in 2009.
Brawn said the more conservative and tense driver in the second half of the season was the result of the "pressures" of the championship battle.
"He's not looked necessarily quicker than Lewis, but he's doing a better job and he's getting the results," he added.
Former long time McLaren test driver Pedro de la Rosa said most paddock regulars did not expect Button to have the upper hand over Hamilton.
"Hardly anyone counted on that," he told the Swiss newspaper Blick.
"He (Button) has stayed calm and had very few problems with the tyres. But I know Hamilton's strengths. He will soon wake up and strike back," de la Rosa predicted.
Indeed, when asked to name his picks for the eventual title battle, the Spaniard answered: "Good question, difficult answer.
"Hamilton, Vettel or Alonso -- and not necessarily in that order!"