Petrol retailers have insisted that their fuel is not faulty, despite hundreds of complaints that cars have broken down after leaving forecourts
Both Tesco and Morrisons said they had carried out tests but had been unable to find any problems with their fuel.
Trading standards officials have launched an investigation after drivers in south-east England said they had been sold "contaminated" fuel.
Motorists believe they may have been sold petrol containing ethanol.
Their say their vehicles have juddered, misfired and suffered a loss of power.
I've been in this industry for over 30 years and I don't remember anything like this happening before
Jonathan Church, Tesco's media director, said the company was testing fuel from a terminal in Essex but had so far been unable to find "an issue".
Morrisons also issued a statement following the complaints. The chain insisted it had found nothing wrong with fuel bought from its stores.
Independent oil company Greenergy said tests on the fuel it supplied to Tesco and Morrisons showed it met industry standards.
Asda said it had received about 20 complaints, all relating to cars from the higher end of the market with fuel management systems.
The company said tests carried out so far had proved inconclusive.
Trading standards said its officials were also testing a number of unleaded samples and the results were expected on Friday.
The penalty for supplying fuel which does not meet industry standards is a £5,000 fine.
The BBC has received more than 2,000 e-mails from motorists who say their vehicles may have been affected.
Most came from an area covering London and parts of east and south-east England and many reported buying petrol from forecourts at supermarkets.
But problems have also been reported by motorists in other areas including Aberdeen, Manchester, Newcastle and Preston.
Tesco and Morrisons have been mentioned most frequently but motorists have also said they bought petrol from Asda and Sainsbury's.
The Association of British Insurers (ABI) said motorists affected by contaminated petrol should be able to claim for accidental damage if they have comprehensive cover.
The problems seem to be centred on oxygen sensors in vehicles. The sensors are attached to the exhaust and determine the mix of oxygen and petrol supplied to the engine.
Garages are running out of replacement parts in some areas as they are swamped with motorists. Repair bills are topping £1,000 for some drivers.
The UK Petroleum Industry Association said the problem could have arisen because of the way a batch of fuel was blended or stored.
A spokesman said the problem appeared to be with a localised batch of fuel and petrol across the UK was generally not faulty.
Ray Holloway, of the Petrol Retailers Association, said garages had told him the problems may have been around for a week, so the exact cause might not be found.
"I've been in this industry for over 30 years and I don't remember anything like this happening before, and therefore to give you a cause is very difficult at this stage."
Michelle Shambrook from advice service Consumer Direct told the BBC that motorists who think they have been affected must have proof of purchase to be able to make a claim and should keep receipts.
They should take their vehicles to an independent garage to have the contamination confirmed.