Stevenage's Lewis Hamilton will join Mercedes from next season on a three-year contract.
The decision to leave McLaren after a 14-year association with the team has been described by one source close to Hamilton as "the toughest of his career".
The 27-year-old replaces Michael Schumacher at Mercedes, with the 43-year-old seven-times champion now likely to retire from Formula One for a second time.
"It is now time for me to take on a fresh challenge and I am very excited to begin a new chapter racing for the Mercedes Formula One Team. Mercedes-Benz has such an incredible heritage in motorsport, along with a passion for winning, which I share. Together, we can grow and rise to this new challenge. I believe that I can help steer the Silver Arrows to the top and achieve our joint ambitions of winning the world championships."