Petrol believed to be at the root of problems suffered by thousands of motorists across the south east of England has been found to contain high levels of silicon.
Fuel from petrol stations operated by supermarket chains Tesco and Morrisons was at the heart of the problem, though other retailers were also affected.
All of the affected petrol stations source their fuel from the same depot in Essex, run by Dutch company Royal Vopak.
Tesco's own tests revealed "significant silicon contamination" in the fuel.
As a result, Tesco is to remove all unleaded petrol from 150 of its petrol stations in the south east, though it will restock and continue to supply petrol.
Morrisons earlier stopped selling fuel at 41 of its petrol stations supplied by Vopak.
Silicon is used as an anti-foaming agent in diesel fuel, but should not be in petrol. Cars that have filled up with the contaminated fuel have suffered from pinking and stalling, and many have had to be taken to garages on recovery trucks.