Jenson Button has been confirmed as a McLaren driver for 2010.
Although the length and terms of his deal have not been confirmed, reports suggest the world champion has signed a three-year deal, on a salary of £6 million a season. That is around 50 per cent more than Brawn/Mercedes were offering.
"It's always a difficult decision to leave a team when you've been there for so long," said Button. "But life is all about challenges - and, most important of all, it's about challenging yourself.
"So, although I won the world championship with Brawn GP last year, and I'll never forget that, I was always adamant that I wanted to continue to set myself fresh challenges. So that's why I've decided to join Vodafone McLaren Mercedes.
"You can't help but be affected by this team's phenomenal history. McLaren is one of the greats of world sport, and its achievements and list of past champions read like a Who's Who of Formula 1: Emerson Fittipaldi, James Hunt, Niki Lauda, Alain Prost, Ayrton Senna, Mika Hakkinen and of course my new team-mate, Lewis Hamilton. I've followed the McLaren team ever since I was a small boy, and it feels unbelievable to finally be a part of it."
Button's move ends a seven-year relationship with his previous team, which he joined when it was still known as BAR in 2003 and stayed with through its difficult seasons under Honda ownership prior to this year's fairytale success.
He said he had been deeply impressed by McLaren since beginning talks with the squad.
"When I visited the McLaren Technology Centre earlier this month, it wasn't simply the technical resources and the incredible standards of excellence that impressed me," Button said. "I was equally struck by the ambition, the motivation and the winning spirit that flow through everybody there. And then there's the team's epic history; put it this way, the trophy cabinets seem to stretch for miles.
"From a personal point of view, it's also a great pleasure to be joining a fellow British world champion. Lewis has achieved an incredible level of success in a very short period of time, and he's a wonderfully gifted driver who has earned the respect of every Formula One driver. I'm sure there's plenty that we can learn from each other, and I'm really looking forward to using our combined knowledge to push the team forward."
With Button no longer in the frame, Nico Rosberg is expected to be announced as Mercedes' lead driver for its new works F1 assault with the erstwhile Brawn team, with Nick Heidfeld linked to the second seat.
Button replaces Heikki Kovalainen at McLaren.