Triple world champion Sir Jackie Stewart believes Jenson Button has been unwise to move from Brawn to McLaren.
McLaren announced earlier today that it would pair the last two world champions next year, having signed Button alongside its current lead driver Lewis Hamilton.
Stewart reckons Hamilton's long relationship with McLaren will make him very hard for Button to beat.
"It will be difficult for him to take on Lewis Hamilton," Stewart told BBC Radio Five. "It's like walking into the lion's den, because Lewis has had three years with McLaren now. He has it his way.
"He's already disposed of one driver that came in there in Alonso. Fernando couldn't cut it against Lewis Hamilton, and I think that Lewis will want to retain his position of prominence in that team. Whereas at Brawn, I think [Button] would have had an advantage over his [potential] team-mate Nico Rosberg, having knowledge of the team.
"What we're looking at today is two guys who want to be number one. That situation in one team sometimes makes imperfect bed partners, because there will be differences from time to time, even though McLaren are probably the best engineering team with the most capacity to do the job."
Button had looked set to stay at Brawn for next season, but could not agree terms with the world champion team, which has now been taken over by Mercedes.
Stewart thinks Button underestimated the value of the strong relationship he had established with Ross Brawn's team.
"I think it's a mistake by Jenson," he said. "If I was Jenson, I would've wanted to do a deal with Brawn - because I know the culture of the Brawn team, I have high respect for Ross Brawn, I know the mechanics.
"There's a totally different culture in McLaren, something that he might never have experienced before. They have a very clinical culture which doesn't have the emotion or the drive in the same passionate way that Brawn would have had with him there as the reigning world champion."
But Stewart acknowledged that McLaren will provide Button with a very competitive package for 2010.
"I do believe that the McLaren car next season will be one of, if not the, best car on the grid, as Ferrari will also be better next year," said Stewart. "So he's going to a team that wants to succeed."