Despite insisting both drivers will receive equal treatment, however, the Woking-based team will pay Button an annual salary of £7.5m – around half what they pay Hamilton.
The first pairing of British world champions since Graham Hill partnered Jim Clark at Lotus in 1968 will have the team's marketing and commercial departments – and those of their sponsors – rubbing their hands.
Button's defection to McLaren came as a surprise to Mercedes Grand Prix (previously Brawn), whose team principal, Ross Brawn, only recently dismissed speculation that his driver was about to leave. They are said finally to have offered similar money to McLaren, but Button's recent visits to the latter helped to swing his decision to leave, even though some believe that Hamilton will prove his nemesis.
As late as Tuesday, Brawn said he would be "amazed" if Button had agreed terms with McLaren. But Button and his management were understood to be furious that, following the takeover, Brawn and team chief executive, Nick Fry – both of whom, as shareholders in the team, personally made millions from the deal – refused to raise their salary offer to Button of around £4.5m a year.
Although that offer included the freedom to sign as many personal endorsements as he liked, the difficult economic climate, combined with Button's belief that McLaren are likely to have a faster car next season, made him decide to take the risk.
The question of whether Button can live with Hamilton – hugely talented and ambitious on the track, and who has a long-standing working relationship with the McLaren support team – will remain unanswered until the 2010 season gets under way in March.
Many have expressed doubt – the bookmakers immediately made Hamilton a firm favourite to outscore his new team-mate next season – but Button has been assured that no favouritism will be shown to Hamilton within in the team.
That message has also got across to Hamilton, who made a pointed reference to "working with [Button] and our engineers to make sure we kick off the 2010 season with a car competitive enough to win the world championship. Although we'll be pushing each other hard, I'm sure we'll very quickly establish a great working relationship," said Hamilton. "He's an exceptional driver: very controlled and very smooth, and he has a real depth of knowledge and experience.
"I sincerely hope we can make the whole of the United Kingdom, as well as Vodafone McLaren-Mercedes fans across the world, proud. Nothing means more to me than to be able to represent my country, and I'm looking forward to both of us painting Formula One's circuits red, white and blue for many years to come."
Button was equally effusive, describing Hamilton as "a wonderfully gifted driver who has earned the respect of every F1 driver.
"It's always a difficult decision to leave a team when you've been there for so long," added Button, who had been with Brawn in their previous incarnations as BAR and Honda for seven years. "But life is all about challenges, and most important of all, it's about challenging yourself.
"You can't help but be affected by this team's phenomenal history. McLaren are one of the greats of world sport, and their achievements and list of past champions read like a Who's Who of Formula One – 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 finally to be a part of it."
McLaren made it clear they chose Button, who turns 30 in January, because he fits the bill as a fast, experienced and talented driver who can help them to win the world championship for constructors, not because he came cheap as negotiations with the newly Mercedes-badged Brawn team floundered.
The team principal, Martin Whitmarsh, said the move was in keeping with McLaren's philosophy of employing the two best possible drivers, and Button was chosen in preference to the 2007 champion Kimi Raikkonen, and Mercedes protégé Nick Heidfeld who drove for BMW Sauber this season.
Running two stars has historically proved painful for the Woking-based team. The enmity between Ayrton Senna and Alain Prost in 1988 and '89 created one of the greatest sporting rivalries of all time and resulted in Prost moving to Ferrari in 1990.
Alonso's partnership with Hamilton in 2007 arguably cost each of them their chance of the title that year, and created so much acrimony that none of the parties like to speak of it to this day. Nevertheless, Whitmarsh is adamant that the situation can be suitably managed. "I'm confident that we'll be able successfully to balance and harness Jenson's and Lewis's complementary skill-sets," he said.
Within the team, however, there are concerns how Button and his camp will fit in with Hamilton and his team, led by his father Anthony who likes to take a much more active role in operations than Button's father John ever has during his son's lengthy career.
McLaren's engineers have also been impressed by the drivers' analysis of Button's qualifying and race data from 2009, which is more important to their racing psyche than any marketing impact the "Dream Team" will create.
Mercedes have already signed German Nico Rosberg, whose place at Williams was taken by Button's 2009 team-mate Rubens Barrichello. But now, after prevaricating too long, they find themselves faced with the likelihood of running Heidfeld alongside Rosberg. Both are perceived as quick drivers, but not quite from the top drawer. Neither has yet won a race, despite 239 race starts between them, and Mercedes may yet come to regret the global publicity opportunities they will forego after losing their champion.
Raikkonen, meanwhile, will take a year's sabbatical and try his hand at rallying for 2010 before seeing how things shake down in 2011.
Tale of the tape: How the two world champions measure up
Lewis Hamilton
Age: 24
Born (Lives): Stevenage, Hertfordshire (Luins, Switzerland)
Family: Father and manager Anthony has been at his side throughout his career, while brother Nicholas is also seen at race days.
Not-so-private life: Pussycat Doll girlfriend Nicole Scherzinger regularly appears at races. Received criticism after moving to Switzerland to avoid taxman.
Route to F1: From radio-controlled cars, he drove go-karts at age of six. Progression through karting ranks saw him get McLaren contract aged 13. Mass of junior titles earned him F1 drive in 2007.
Races: 52 (debut: 2007 Australian GP)
Wins (Poles): 11 (17)
Points: 256
Career earnings (est.): £30m
Jenson Button
Age: 29
Born (Lives): Frome, Somerset (Monaco)
Family: Father John, a former rallycross driver, attends every grand prix. Also has three older sisters.
Not-so-private life: Previously engaged to Louise Griffiths, now dating model Jessica Michibata. Accused of over-indulging in playboy lifestyle.
Route to F1: After taking British and European karting titles, won the British Formula Ford Championship in 1998. Competed in Formula Three before joining Williams in 2000.
Races: 172 (debut: 2000 Australian GP)
Wins (Poles): 7 (7)
Points: 327
Career earnings (est.): £60m
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