Thousands of motorists have suffered breakdowns due to suspected "contaminated" petrol sold in supermarket forecourts.Trading standard officials are now investigating the unprecedented incident as garages struggle to deal with the influx of cars.Drivers said they had experienced problems after buying petrol from Tesco outlets in the south of England, although there were reports it could have spread across the country.The Society of Motoring Manufacturers (SMMT) stressed the urgency of finding the source of the potential contamination, which was still unclear.Its chief executive Christopher MacGowan said: "It appears there is a rogue load of petrol in the United Kingdom at the present moment but until we actually can track down exactly where it has come from and also what the precise problem with it is, it's very difficult to give meaningful advice to drivers."The petrol has been affecting oxygen sensors, which are part of the engine management system.The problem had come "completely out of the blue", he said, adding: "I have been in this industry for many years and I cannot remember a similar occurrence on this scale."An investigation was launched after complaints some service stations, including some at supermarkets, had been selling petrol contaminated with ethanol without telling customers.Ian Hillier, from the Trading Standards Institute, said: "It is illegal to do this and trading standards authorities in the affected areas have begun an inquiry."He said he understood there to have been around 75-100 complaints from people in the south east of England.Tesco said that it had done extensive tests on batches of fuel and had not found any link back to the store, but was working "urgently" to find out what had happened.© Independent Television News Limited 2007. All rights reserved.