Supermarkets are clearing pumps of contaminated petrol blamed for causing thousands of cars to break down.
Specialist contractors have been hired to help. Picture by Paul Wood
One industry group said the incident could force prices up by 2p a litre.
Earlier, supplier Harvest Energy said "unusually high levels of silicon" had been traced to four storage tanks at a depot in Essex.
It had not been detected before sale at retailers including supermarkets Tesco and Morrisons because routine tests did not look for the substance.
The faulty unleaded petrol has been blamed for damaging thousands of cars in south-east England.
More than 5,000 people have contacted the BBC to say they have been affected.
Harvest Energy shares tank facilities at the Vopak Terminals depot in West Thurrock with another oil company, Greenergy, which supplies both Tesco and Morrisons.
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
In a statement, Greenergy said a component understood to have been used by Harvest Energy in the production of unleaded petrol had tested positive for excess levels of silicon.
Morrisons has stopped selling unleaded fuel at 41 outlets supplied by the depot until tests are completed.
Tesco said it was emptying its unleaded tanks at 150 outlets in south east England and refilling them with uncontaminated fuel, but continuing sales.
Asda, which buys fuel from Harvest Energy, said it was replacing unleaded petrol at 30 forecourts in the south east supplied from the affected depot as a "precautionary measure".
Harvest Energy said it could guarantee no further contaminated petrol would be distributed.
An independent inspection company would be testing fuel stocks at more than 100 petrol stations.
"We are very sympathetic to the plight of motorists who have been affected by this problem with unleaded petrol," said Harvest Energy's managing director, Franco Bussandri.
"We are working as closely as we can with our customers the retailers, to address such motorists' concerns."
"We would ask motorists who feel they may have been affected to contact their fuel retailer for advice."
Ray Holloway, from the Petrol Retailers Association, told BBC News 24 prices could be affected.
"The need to buy short-term replacements for this damaged fuel will have been noticed by the traders on the markets," he said.
"The cost price has been pushed up so it's very likely that motorists will notice on their forecourts in the next couple of days, a couple of pence more per litre."