Scientists last night said they had found silicon in fuel suspected of causing thousands of cars to break down during the past week.
Trading standards officers said they had carried out spot checks on fuel from forecourts in Cambridgeshire, Kent, Essex and Buckinghamshire in an attempt to identify any contaminant.
Thousands of motorists had reported a similar fault on their cars in recent days which experts thought had been caused by contaminated fuel.
Many motorists experiencing problems say they bought fuel from filling stations at Tesco and Morrison's supermarkets.
The problem batch of fuel has been linked to a Thames Estuary depot which supplies several fuel companies and at least three supermarket chains. Industry experts suspected that silicon, or its compound form silicone, may have been responsible for contaminating a batch of petrol supplied to forecourts run by Tesco and Morrison's.
Kent County Council Trading Standards said they had not received test results yet and were expecting more information next week.
Traces of the chemical had been found in petrol taken from some stricken cars.
Experts fear that the contaminant is damaging delicate oxygen sensors causing engine management systems to switch to emergency settings.
Certain makes and models have appeared to be more vulnerable.
Renault, Peugeot, Mercedes Benz and Citroen said they have made special orders for more sensors to replace damaged components.
Owners of Ford, Volkswagen, Audi and Seat brands seem to have experienced no problems.
Honda, Volvo and Porsche also have reported little, if any, extra demand for parts.
Motorists have reported paying more than £1,000 to replace damaged sensors, but the AA said a typical bill should be about £200.