Test results on fuel which is thought to have caused problems for thousands of drivers in the south and east of England are expected to be released
The petrol has already been traced to a storage depot in Essex.
A Trading Standards report is to be released this morning, with another by the UK Petroleum Industry Association due in the afternoon.
A source at the AA says traces of silicon has been found in petrol taken from some of the affected cars.
'False signal'
Tesco confirmed that it is testing for traces of silicon, but previous tests by the supermarket chain have failed to find any contamination.
It is thought silicon may have found its way into a batch of petrol in transit by ship into the UK or in storage containers
The problems seem to be centred on oxygen sensors which determine the mix of oxygen and petrol supplied to the engine.
Automotive expert Professor Malcolm Fox, from the University of Leeds, said all the signs pointed towards silicon having got into the fuel.
He added: "The analysis from the suppliers shows that the fuel is okay, so it must be a very minor, low concentration contaminant.
"When it burns, it will form silicon dioxide and will deposit a very thin sheet on the sensor - the sensor is in the exhaust - and that probably gives a false signal to the engine computer."
Chris Hunt from the UK Petroleum Industry Association (UKPIA), said: "Silicon is one of the many substances we will be examining in the samples we have yet to evaluate, and that evaluation will be going on over the next 24 hours."
The UKPIA 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.
Contamination by water or ethanol has now been ruled out.
Tesco said the problems appeared to be linked to forecourts in the south-east of England served by the Vopak Terminals depot in Essex.
Both Tesco and Morrisons get fuel from an independent oil company called Greenergy, which insists its supplies meet industry standards.
'Deviation'
Vopak Terminals managing director Colin Scott said: "Verifications are continuing. At this moment we have not established any deviation from normal procedures."
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 several thousand e-mails from motorists who say their vehicles have juddered, misfired and suffered a loss of power.
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.
First published on the BBC Website|