A mystery outbreak of car engine problems - possibly the result of contaminated unleaded petrol - has mystified authorities and the motor and fuel trades in parts of England.

Reports of cars spluttering and stalling after refuelling at independent service station or supermarket outlets began appearing in media in London and the east and south of the country yesterday (28 February).

Service technicians had reported numerous failures of exhaust system sensors and there was also a consequent shortage of replacement parts.

The Society of Motor Manufacturers and Traders (SMMT) suggested possibly faulty fuel was contaminating exhaust system sensors which then shut down engines to protect them. The trade group added that automakers were working to rush replacement stock to retailers.

So far, the problem has not been identified by regional Trading Standards officers who have been testing fuel samples. Meanwhile, a trade group which represents oil company outlets said they had had no problems with their fuels.

A spokesman for the Tesco supermarket chain said it was investigating complaints from drivers who had refuelled cars at its service stations but had not so far identified any problems with the petrol.

"We are aware that customers of other supermarkets have also reported difficulties," a Tesco spokesman told Reuters, adding: "Whilst we cannot currently trace any problem back to Tesco fuel we will of course continue to urgently work with our supplier to identify what might be behind it."

Independent oil firm Greenergy told the news agency it was examining whether the problem was related to its supplies.

"So far we have conducted extensive tests on the fuel supplied to Tesco and Morrisons [another supermarket chain]," it told Reuters in a statement. "We have found it is fully compliant with BSEN 228 - the independent standard everyone works to."

There have been suggestions ethanol may have accidentally been added to the fuel mix but this has not been confirmed by any investigation so far. Most recent car models sold in the UK can handle a mix of up to 5% ethanol in their petrol and there are also strict rules here regarding the labelling of pumps dispensing bioethanol mixes.

Fuel-related problems temporarily hit New Zealand motorists about 25 years ago. After the local refinery shared by four oil companies was shut for major maintenance, alternative petrol supplies were sourced from refineries in Australia and Asia.

One company experienced problems after motorists alleged their cars developed fuel system leaks after refuelling at its petrol stations. Although the fuel supplier eventually settled some damage claims, many motorists found the problem simply went away after their next petrol tank refill.

A batch of aromatics - used to enhance the performance of the then-leaded fuel - was the key suspect but the cause of the leaks was never fully identified.