Formula One's governing body has warned Michelin teams that they face heavy sanctions if they race in Sunday's U.S. Grand Prix with different tyres to the ones they qualified on.
Michelin supply seven of the 10 teams, including championship leaders Renault and leading rivals McLaren.
Their preparations for Sunday's ninth round of the championship have been thrown into confusion by two crashes in Friday practice involving the Toyota team and caused by sudden tyre failure.
Michelin said on Saturday that they did not know the cause of the failures and would have to advise teams not to race with the tyres they qualified on.
However, under rule changes this season, teams must qualify and race with the same tyres.
"Some of the teams have raised with us the possibility of running a tyre which was not used in qualifying," the governing International Automobile Federation (FIA) said in a letter to Michelin bosses.
"We have told them this would be a breach of the rules to be considered by the stewards.
"We believe the penalty would not be exclusion but would have to be heavy enough to ensure that no team was tempted to use qualifying tyres in the future."
FUTURE ACTION
The FIA criticised the French manufacturer for failing to ensure that they had a safe alternative for use at Indianapolis and raised the prospect of them being called in front of the World Motor Sport Council.
Judging from precedents, that could entail a severe sanction.
Michelin, in an earlier letter to the FIA, said it had informed teams "that we do not have total assurance that all tyres that qualified the cars can be used unless the vehicle speed in turn 13 can be reduced."
The FIA said it understood that the French manufacturer had told teams the rear left was safe for a maximum of 10 laps at full speed.
It expressed surprise at the situation, given Michelin's years of experience at Indianapolis.
The final banked turn into the circuit's long pit straight puts huge pressure on the rear left tyres, with cars exiting at around 300 kph.
The FIA asked Michelin to advise teams of a safe speed for turn 13 and to make sure that other cars were not impeded.
It said another option would be for teams to change repeatedly the affected tyre during the race.
"If the technical delegate and the stewards were satisfied that each change was made because the tyre would otherwise fail (thus for genuine safety reasons) and that the relevant team were not gaining an advantage, there would be no penalty," declared the FIA.
"If this meant using tyres additional to a teams' allocation, the stewards would consider all the circumstances in deciding what penalty, if any, to apply."
The FIA dismissed as "out of the question" suggestions that a chicane should be laid out in Turn 13 for safety reasons.
"To change the course in order to help some of the teams with a performance problem caused by their failure to bring suitable equipment to the race would be a breach of the rules and grossly unfair to those teams which have come to Indianapolis with the correct tyres," it said.
