Sir Jackie Stewart reckons Lewis Hamilton and Jenson Button are living in fear of being "beaten up" by Michael Schumacher in this season's Formula One world championship.

Seven-time champion Schumacher will be the centre of attention on his return to the sport at next weekend's Bahrain Grand Prix at the age of 41.

The German's decision to end his three-year retirement means four world champions will be on the grid this season.

Stewart, Britain's most successful F1 driver with three titles, said: "It's fantastic for motor racing that he's come back because he is now the biggest focus of interest.

"Fernando Alonso driving a Ferrari is going to be tough to beat. You've got Jenson in there as world champion; you've got Lewis, who wants the world championship back.

"To see that occurring with Schumacher back, that's the extra edge - in my opinion - for them. Do they really want to be beaten up by a guy of 41 years, who's already been out of the sport?

"He's going to come back and say, 'I know how to do this, and I certainly know how to thread the needle'."

Stewart believes Schumacher's Mercedes team are already playing catch-up to their rivals, with the German manufacturers not taking over Brawn until after this season's car had been in development.

"Maybe it'll take him another year to get the car up to speed," Stewart said. "It may not be as good a car as Ferrari and McLaren are producing this year.

"Schumacher's so experienced and he's so good at co-ordinating people that I think you'll see the performance improving for him.

"I could see him not being up at the front row to begin with, but you certainly couldn't count him out."