The US GP is under threat hours before the start, as the dispute over the safety of Michelin tyres is unresolved.
Michelin had suggested the seven teams they supply might not race unless they could explain the tyre failure that lead to Ralf Schumacher's Friday crash.
F1 officials have turned down a request for a chicane at the accident spot.
That leaves the teams four options; take the corner slowly, stop frequently to change tyres, run new tyres (risking serious penalties) or pull out.
That would have left only six cars on the grid - Ferrari, Jordan and Minardi (who all use Bridgestone tyres) - which could see the race abandoned.
Championship leaders Renault, McLaren, BAR, Williams, Red Bull, Toyota and Sauber all use Michelin tyres.
If the race goes ahead as planned, it is scheduled to start at 1900 BST.
The crisis begun on Friday after Toyota driver Schumacher smashed into a wall in practice and despite tests, Michelin were unable to "understand or reproduce" the tyre failure that caused the crash.
The company wanted to fly in new tyres from its factory in France, but International Automobile Federation (FIA) rules introduced this season, prohibit the introduction of a new tyre part-way through a Grand Prix weekend.
If teams did race using alternative tyres, they would be penalised.
They had also suggested the introduction of a chicane on the banked final corner, where the crash happened, a proposal rejected by FIA Charlie Whiting race director.
"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 grossly unfair to those teams which have come with the correct tyres," he said.
This leaves the Michelin-supplied teams with three other possible courses of action.
They could deliberately slow down on the final corner, make repeated pit stops to change troublesome tyres, or run different tyres in the race from the ones they used in qualifying, running the risk of serious penalties.
"We have told them this would be a breach of the rules to be considered by the stewards," said a letter sent by Whiting to Michelin.
"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."
So with just hours to go before the race, it could still be abandoned or mass disqualification could take place.
Renault chief Flavio Briatore said his team would not take part in the US GP if Michelin advised them not to.
"We fully support Michelin in this situation and in the efforts they are making to best resolve it," he said.
"They are flying out different tyres to use in the race, the same that were used for the Spanish Grand Prix.
"The authorisation to use them has not yet been granted. If that were not given, we would not compete."
Championship leader Fernando Alonso also supported Michelin's stance - but said he thought it was unlikely a boycott would be necessary.
The Renault driver said: "We are worried - all of the Michelin teams are.
"If Michelin say it's not safe, the drivers would agree not to race, but this is an extreme option and this will not happen, never.
"I think this low (tyre) pressure some of the teams ran yesterday was some of the problem," he added.
Schumacher's team-mate, Ricardo Zonta, also suffered a similar blowout at a less critical part of the circuit.
Only two of the 14 drivers using the French company's tyres set a timed lap in the first 45 minute session, the rest diverting through the pit lane rather than taking the final, banked curve flat out.
However, all went out in the second, with McLaren's Kimi Raikkonen setting the quickest time ahead of BAR's Jenson Button and Alonso.
"There are many tyre problems across the pit lane," Alonso added.
"I think it's going to be a very risky race to start off with."
Michelin said they had been working with their experts in Clermont-Ferrand, France, to try to find a tyre that they knew would be safe for Indianapolis.
Toyota's Jarno Trulli qualified fastest on Saturday, ahead of McLaren's Kimi Raikkonen, with BAR's Jenson Button in third place.