Tyre controversy set to rock F1

It is understood that motor racing's governing body, the FIA, has written a letter to all the teams this week claiming that it will change the way it measures tyres from the next grand prix at Monza on September 14.

In the letter, the FIA explains that the previous tread width limit of 270mm for the front tyres will now be measured after the race - rather than before it, as had been the previous practice.

It is believed that this new ruling will hurt Michelin more than Bridgestone because its tyres have previously used the 270mm limit to the maximum.

Confirming the FIA's decision, Williams' chief operations engineer Sam Michael told AUTOSPORT: "The FIA have changed the way they measure the front tyre contact patch. They will now be measuring the contact patch new as well as worn. We are reviewing the consequences of that."

The new ruling is believed to have come after complaints from Michelin's rivals - who claim that when the French tyre manufacturers' rubber wears down during a race the tread width of the tyre exceeds the 270mm limit. There may also be instances that the tread width exceeds the limit when the tyre is running at speed, although this is difficult to prove.

Although Michelin is expected to fight the decision, because it claims the tyre design has been fully legal since it was first used at the 2001 San Marino Grand Prix, it could be forced to build completely new designs for Monza.

There may not be enough time for Michelin to prepare these tyres in time for the pre-Monza test next week - which means Williams and McLaren could race with tyres at the Italian event which have not been tested before.

A spokesman for the FIA said: "A letter was sent to Formula 1 teams regarding tread width, but we cannot say any more at the moment."