Two supermarket chains at the centre of the "faulty fuel" alert have moved to stop supplies of suspect petrol.
Unleaded petrol has been withdrawn from 41 Morrisons outlets
Morrisons has stopped selling unleaded at 41 outlets supplied by the Vopak depot in Essex - at the centre of suspicions over fuel.
Tesco, which found silicon in its fuel, is to empty its unleaded tanks at 150 outlets in the South East, but will refill them and continue sales.
More than 5,000 people have contacted the BBC to say they have been affected.
After testing its own fuel, Tesco said in a statement: "These tests show evidence of some significant silicon contamination in samples taken from unleaded petrol released from the Vopak terminal around 10 days ago.
"We believe this to be the source of contamination subsequently experienced by some customers in the south-east of England."
SILICON & SILICONE
Silicon is a chemical element - symbol 'Si'
Silicone is a chemical compound containing silicon and oxygen
Silicone can take solid or liquid form and is used as a lubricant and insulator
When silicon is found by tests on petrol it may indicate the presence of silicone
Other tests on petrol taken from the tank of a car affected by allegedly faulty fuel have detected the presence of silicon, trading standards officers said earlier.
Cambridgeshire County Council's trading standards department had the fuel sample tested at a London forensic science laboratory.
A spokesman said silicon in the fuel could indicate the presence of silicone - a class of chemical compounds containing the element.
Such products are used in diesel as anti-foaming agents, but can cause "serious problems" in petrol engines, a trading standards official said.
Faulty unleaded petrol has been blamed for damaging thousands of cars in south-east England.