Petrol retailers have insisted that their fuel is not faulty, despite hundreds of complaints from drivers that their cars have been damaged.
Thousands think they have been affected
Both Tesco and Morrisons said they had carried out tests but had been unable to find any problems with their fuel.
Vopak, a fuel distribution company with a depot in Essex, said it was checking that it was not to blame.
And trading standards officials said their tests on the "contaminated" fuel may be available on Friday.
Motorists believe they may have been sold petrol containing ethanol.
They say their vehicles have juddered, misfired and suffered a loss of power.
Both Tesco and Morrisons get fuel from an independent oil company called Greenergy, which insists its supplies meet industry standards.
The main Greenergy storage depot is on the River Thames at West Thurrock, Grays, Essex.
One of the fuel distribution terminals there is run by Vopak, which specialises in handling liquid chemical and oil products.
Colin Scott, Vopak Terminals managing director, said checks were being made to make sure normal handling and storage procedures have been followed.
"Verifications are continuing. At this moment we have not established any deviation from normal procedures."
Morrisons has also issued a statement insisting that it had found nothing wrong with fuel bought from its stores.
Asda said it had received about 80 complaints, many relating to cars from the higher end of the market with fuel management systems.
The company said tests carried out so far had found nothing wrong.
The Trading Standards Institute said samples of unleaded petrol had been taken from station forecourts in Cambridgeshire, Kent, south Essex and Milton Keynes.
A spokeswoman said spot checks were also being undertaken by local authority trading standards teams in other parts of the country.
The penalty for supplying fuel which does not meet industry standards is a £5,000 fine.
The BBC has received more than 3,500 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.
The Association of British Insurers (ABI) said motorists affected by contaminated petrol should be able to claim 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 single batch of fuel was blended or stored, and that 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."