UK supermarkets have already cranked up their own petrol price war in the wake of falling oil prices. Sainsbury's and Asda are planning to cut the price of diesel by up to 2p a litre and unleaded petrol by a penny from tomorrow. Tesco, which is Britain's biggest petrol retailer with about 500 petrol stations, will cut unleaded petrol by 1p per litre, with diesel reducing by at least 1p a litre although some sites will get a 2p a litre price cut.