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"It was a very special feeling that I had achieved what I set out to achieve in Formula One. But immediately I thought 'So, what happens now?' Where do I go from here?," said the driver, who had been with the team through various incarnations as BAR/Honda/Brawn since 2003.

"I've been very comfortable within that team (Brawn) and it is taking me outside of my comfort zone," said the 29-year-old, in London to publicise his new book "My Championship Year", of the dramatic move.

"But that's what excites me and that is a real challenge for me. That is a challenge I am definitely up for. It's going to be a challenge going to a team where Lewis has been for three years and where he's won the world championship," added Button, whose former team has now been taken over by Mercedes. "That's why I am doing it. I want to challenge myself, I want to challenge Lewis and that is so exciting for me. That's the reason why I am going, I'm not going because it's going to be easy, I'm going because it's going to be tough."

Hamilton, who became the youngest ever world champion in 2008, has been backed by McLaren for more than a decade and the 24-year-old has built the Mercedes-powered team around him.

Britain's retired triple world champion Jackie Stewart warned on Wednesday that Button would be stepping "into the lions' den" and should have stayed put.

"You can say 'Well, isn't winning a second championship a big enough challenge?," mused Button. "Well, it is. But I wanted something else. It was only a couple of weeks ago that I knew it was a possible option and it made me very excited. It definitely came from the heart, that decision to move to McLaren and to be fighting with Lewis Hamilton in the same car."

Button said Formula One fans should be excited by the prospect of an all-British line-up of champions, and had no time for the idea that it would be better to have an easier partner.

"I think its fantastic. Why should one driver have the help of another? That's just not the way it should be in Formula One," he said. "You should have two competitive drivers and they should be fighting it out between them and that does move the team forward. A team needs two drivers to be quick, to be competitive, to work and both drivers should always be given equal opportunities."

Hamilton's pairing with Spain's double world champion ended in tears for Fernando Alonso in Hamilton's debut 2007 season, while Frenchman Alain Prost and Brazilian Ayrton Senna were at each other's throats at McLaren in the late 1980s.

Button said Hamilton would not be his first tough team mate, recalling his time with Canada's 1997 world champion Jacques Villeneuve at BAR, and was sure he could handle the situation.

"I want to race against one of the best drivers in the world and he is going to try and beat me. I will try and beat him. That's the way it is," he said.

The Briton said he had spoken to Ross Brawn, his former team principal on Wednesday morning and explained his decision.

"It's always disappointing to go your separate ways when you have been so close to someone who has really helped you to win a title," he said, "but I think he understood my reasons behind it and we will stay friends, I hope."

The Briton added that his visit to McLaren last Friday, widely seen as a key to his decision, had not been fundamental.

"I had feelings that I wanted to be fighting there alongside Lewis before that meeting and it wasn't that day at the factory that clinched it for me," he said. "It was at another time and place."

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