Ferrari, overtaken in the championship by Williams, have said they alerted Formula One's governing body to suspicions that their rivals were using illegal tyres.
Ferrari technical director Ross Brawn told the Gazzetta dello Sport on Wednesday that they contacted the International Automobile Federation (FIA) after receiving photographic evidence at the Hungarian Grand Prix.
Williams and McLaren, Ferrari's main rivals, both use Michelin tyres.
"It all started in Budapest when Bridgestone got hold of some photos taken in the paddock by a Japanese," said Brawn at the Monza tests.
"They showed in an unequivocal way that the front Michelin tyres had an excessively large tread at the end of the race or after being used.
"We turned to (race director) Charlie Whiting who, on the basis of his measurements and the photos, then sent the famous letter.
"We could have pretended to have seen nothing, to not let the FIA know and then lodge a complaint at the following race, but that didn't seem like the right thing to do."
Whiting wrote to grand prix teams last week warning them that the FIA would check tyres closely after races to ensure that no more than the maximum permitted amount of tread had made contact with the road.
Michelin insist they have been using legal tyres but have sent mixed messages about what they intend to do for next week's Italian Grand Prix, Ferrari's home race.
CRITICAL TIME
The race at Monza comes at a critical time in the championship, with Ferrari's reigning champion Michael Schumacher leading Williams' Juan Pablo Montoya by one point with three races remaining.
McLaren's Kimi Raikkonen is another point back. Williams are eight points ahead of Ferrari.
Michelin motorsport boss Pierre Dupasquier told the company's website that Michelin were responding to the challenge.
"Our people have been working 24 hours a day since last Wednesday and new tyres are being tested in Monza as we speak," he said. "For anyone who knows anything about the tyre world, to do what we've just managed is a huge achievement."
On Tuesday however, Michelin's Formula One programme manager Pascal Vasselon had said that there was no reason to change the tyres and no time to change them for the tests either.
Brawn said the front tyre tread advantage, taken into consideration with the chassis, could account for half a second of performance gain.
But asked whether Michelin's tyres were enough to explain Ferrari's below-par performances, with Schumacher lapped in Hungary by Renault's race winner Fernando Alonso, Brawn replied:
"You can't say that. But it's clear that if you take an advantage like that away from the competition you'll find yourself in a better position."
Brawn said he doubted whether Ferrari would need to make a protest after the race at Monza.
"I hope it does not come to that, that Michelin realise that they have enjoyed an illegal advantage for so long, too long, and that they conform to what the FIA sets out. In any case, it's a problem for the sporting authorities."
