Trading standards officials have launched an investigation after drivers complained that they had been sold "contaminated" fuel.
Ethanol content should be made clear
Drivers from across south-east England believe they may have been sold petrol containing ethanol, which has damaged their cars.
Their vehicles have been juddering, misfiring and suffered a loss of power.
Independent oil company Greenergy said it was testing fuel supplied to Tesco and Morrisons to see if it was faulty.
Greenergy, which supplies "greener" fuels, said its "extensive" tests on the batch of fuel supplied to Tesco and Morrisons showed that it met industry standards but it was continuing its investigation.
A spokeswoman said the fuel contained up to 5% bio-ethanol, which was within permitted limits.
"So on that basis ethanol does not seem to be the cause of the problem but we are still testing," she said.
The company added in a statement: "Greenergy has been made aware that some drivers using our fuels are experiencing some problems. We take this very seriously."
Trading standards said its officials were also testing a number of samples but had not yet had the results.
Ian Hillier, a petroleum spokesman for the Trading Standards Institute, said: "I understand that there have been around 75-100 complaints from people in south-east England.
"Including ethanol in petrol is not really contaminating it, but there has to be a clear warning to people who buy petrol as to exactly what is in it."
The penalty for supplying fuel which does not meet industry standards is a £5,000 fine.
Tesco said its initial tests had not revealed any contamination, although it would continue to investigate the problem.
Jonathan Church, Tesco's media director, said the company was testing fuel from a terminal in Essex.
"We've tested and re-tested including from some of the vehicles that have had reported problems and we actually can't find an issue.
"So at this stage we can't trace the problem back to Tesco fuel. That said, obviously we're continuing to work with our supplier to see if we can find a problem and if there is one we'll let customers know."
Morrisons also issued a statement following the complaints. The chain insisted it had found nothing wrong with fuel bought from its stores.
"Morrisons suppliers test every batch of unleaded petrol to ensure that it meets British and European standards.
"Having received a limited number of enquiries further tests were carried out. These found no contamination and confirmed our unleaded petrol met the required standards."
AA technical specialist Vanessa Guyll said drivers would notice problems with the running of their cars almost immediately, and providing they sought help quickly, it was unlikely the "contaminated" petrol would cause any long-term damage to engines.
The RAC said the problem seemed to be particularly affecting the oxygen sensors in vehicles made by Vauxhall, Peugeot, Citroen and Mercedes.
Oxygen sensors are attached to the exhaust and determine the mix of oxygen and petrol supplied to the engine.
Christopher MacGowan of the Society of Motor Manufacturers and Traders said thousands of motorists had been affected, most of whom had vehicles that were more than three-years-old.
He said: "I think that we've got a corridor in east Anglia, London and the South East where somehow a dodgy batch of petrol has crept into the system.
"In the context of over 28 million vehicles on the road, you have to keep it in proportion, but it is serious, and it's very very unpleasant for those several thousand drivers who've experienced this problem," he added.
Hundreds of drivers have also been contacting the BBC after filling up their cars with the allegedly contaminated fuel.
Some car dealerships say they are trying to repair damaged cars, many of which are now off the road, but the parts they need are now in short supply.
Motorist Darren Ross, 42, has complained to the Department of Trade and Industry about the problem.
He buys petrol for his Saab convertible 1.8T at the Tesco superstore in Waltham Abbey, Essex, close to where he lives, and believes it led to his car breaking down.
His local Saab dealership told him it was a fault with sensors and that they had had 15 other similar problems over the last week.
Mr Ross said the repairs are costing him £200, which is not covered by his warranty because they are linked to a fuel fault.
"I want compensation and reimbursement. I need my car for my work and it has been back to Saab four times in the last two weeks."