As motorists wait for the results of tests on the "contaminated" fuel, experts speculate on what may have caused the problem.
Contamination can occur during the long journey from refinery to forecourt
Some experts say contamination is more likely to happen when fuel is stored or delivered than during its manufacture.
One of the crucial stages in the lengthy process by which oil ends up as fuel in a petrol tank takes place in a refinery when crude oil is heated in a distillation unit.
The crude oil is heated to produce four main streams which produce jet fuel for planes, diesel, gasoline for making petrol and fuel oil for heating.
In some of these cases sulphur will be removed, in all cases the fuel will be stored at the same refinery in different storage tanks.
The next stage involves transporting the fuel by sea or by road to the underground storage tanks at petrol filling stations.
Dr Richard Pike, chief executive of the Royal Society of Chemistry, said: "It [contamination] is likely to be in the transportation process or storage - because there's so much blending and the fact that, at some time, different fuels can be put into the same storage tanks.
"It's important to clean out the tanks each time there's a change over in the type of fuel. The same also applies to ships and road tankers.
"A good operator will do this as well as tests in the laboratory to check that the quality assurance process is followed."
Clifford Jones, an engineering academic at the University of Aberdeen, suggested three possible explanations - other products could have mixed with fuel at the refinery, an octane enhancer might not have been added to petrol which needed one, or bio fuels were put into cars which were not designed to use them.
Octane prevents the car "knocking" or "pinging" or "pinking" when the air fuel mixture is ignited, damaging the engine.
'Testing round the clock'
The UK Petroleum Industry Association, which represents the UK's nine main oil refining companies, said the cause of the problem may lie with how the unleaded petrol was blended, stored or it may lie with some unknown contaminant.
Nick Vandervell, of the UKPIA, said:"The advice is that this is not a generalised problem. The fuel in the UK is perfectly OK and this seems to point at a fairly localised batch of fuel going through the system which has affected the cars."
He said that reports that ethanol was contaminating the fuel may not be correct, as industry standards permitted 5% ethanol to be added to unleaded fuel and many modern cars could cope with a higher proportion.
He added that because the first indications of a widescale problem were reported last week, the time that had passed could make it more difficult to discover the cause of the problem.
Some of the fuel supplied to Tesco and Morrisons comes through a distribution depot at West Thurrock in Essex, via an independent company oil company called Greenergy - which has its main storage facility in the same area.
Both Vopak, the distribution company, and Greenergy, the supplier, insist that their checks have shown no abnormalities, although they are continuing to investigate.