Formula One's governing body may put the brakes on Juan Pablo Montoya and Kimi Raikkonen's title challenges after questioning the legality of their Michelin tyres.

The situation could throw the most closely fought championship in years into confusion.

Michelin boss Pierre Dupasquier was quoted on Saturday as suggesting that Williams and McLaren could even boycott the next race at Monza, although Formula One sources said that was unlikely.

"It is possible that the five teams using Michelin tyres will not turn up at Monza," he told The Daily Telegraph. "Our partners would have to spend a lot of money without any guarantee that they would not be disqualified. It is up to them to decide."

The governing International Automobile Federation (FIA) confirmed that it had written to teams using the French company's tyres outlining new rules for measuring tread width at the September 14 Italian Grand Prix.

"We have written to all the Formula One teams concerned," a spokesman said, referring to article 77(c) of the Formula One sporting code. "Should anyone be in breach of this article, they will be referred to the stewards at Monza."

Williams driver Montoya is just one point behind Ferrari's five-time world champion Michael Schumacher with three races remaining. McLaren's Raikkonen is a point further back.

Williams, who now lead the constructors' championship, McLaren, Renault, Jaguar, and Toyota all use Michelin tyres while the other teams are on Bridgestone. Monza, near Milan, is Ferrari's home race.

Michelin teams filled the top seven places at last weekend's Hungarian Grand Prix with Schumacher finishing eighth after being lapped by Renault's Spanish race-winner Fernando Alonso.

FIA sources said fresh information came to their attention after that race suggesting that Michelin's front tyres could be in breach of the rules in some circumstances.

While the tyres conformed to the maximum tread 'contact patch' of 270mm when measured statically before the race, there were suspicions that they might become too wide once run at speed.

"We feel there may be systematic use of a part of the tyre as tread that doesn't look like tread when the tyre is submitted for scrutineering," said a source.

The new rule will count as tread any part of the front tyre deemed to have been in regular contact with the track.

Michelin pointed out in a statement on Friday that their front tyre profile had been deemed 'to comply with the F1 regulations' in writing by the FIA.

"We are fully open to discuss this regulation change wished by the FIA and help to define a measurement procedure which could be enforced in 2004," the company said. "We feel it is, of course, not realistic to ask for it before the Italian Grand Prix."

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