One service manager at a Mercedes franchise in Hertfordshire told us that he'd repaired 15 cars with contaminated fuel in the past four days.
"It's terrible," he said. "The number of cars coming in is unbelievable - all of them have fuel that has been contaminated. Half of them came in after they refuelled at Tesco down the road. We don't know whether it is water or something else, but it is clear there is a problem."
"We've had to flush out their fuel tanks and fuel lines and then replace the oxygen sensors," he explained. "Customers are paying between £800 and £1000, and it's a shame for them because their warranties do not cover this sort of thing."
Tracing the root of the problem
Various possible explanations for the breakdowns have already been mooted. Among them are a bad batch of lambda sensors, a batch of fuel with a disproportionate amount of ethanol in it, and a batch of fuel contaminated in some other way.
The first two causes have been widely discredited. First, there is clearly no reason why thousands of lambda sensors, made at different times, would all fail at around the same time, in roughly the same place.
Secondly, Tesco says all the unleaded it sells conforms to British Standard EN228, which allows a maximum concentration of five per cent ethanol in any unleaded petrol. The most likely cause of the problem, experts say, is contamination at the refining stage, or degradation at the distribution stage.
One particular fuel company has been identified in several reports as a possible supplier of the dirty fuel to both Tesco and Morrisons; Greenergy International. It has distribution depots in Grays, London and Dagenham, Essex and, among other fuels, it exclusively supplies Tesco with 99-octane petrol.
According to a statement posted on Greenergy's website, it is doing everything possible to find out what the issue is. "We have conducted extensive tests on the fuel supplied to Tesco and Morrisons and have found that it is fully compliant with BS EN228." The company is continuing its investigation, and is expected to make another statement later today.
The Trading Standards Agency has also launched an official investigation into the situation, and is advising anyone who's been affected by it to keep a sample of the dirty fuel they bought, a copy of the relevant receipt, and to keep receipts for repair work done on any car as a result of any mechanical problem.