Trading standards officials have launched an investigation after drivers complained that they had been sold "contaminated" fuel.
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 had 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."