Industry analysts expressed surprise at the extent of the supermarket reductions and said that the lower prices may not be sustainable for long.

After two previous rounds of cuts beginning almost two weeks ago, Asda declared on Wednesday that it was uniformly reducing pump prices across its 167 petrol stations to 91.9p per litre for unleaded and 93.9p for diesel. Sainsbury’s and Tesco quickly said that they would follow their rival, but could not give a figure to match Asda’s new national price because they both operate local pricing systems.

Most petrol companies also set their forecourt prices locally, and in some places have been forced by neighbouring supermarkets to accept lower profit margins. A Total spokesman said that its policy was to ensure that its stations were “competitively priced in the local trading area”.

Localised battles for custom are, therefore, developing in areas where Asda or another supermarket is depressing prices, leaving motorists outside these regions paying significantly more for fuel.

Yesterday unleaded petrol prices averaged 95.38p a litre, down from a peak of 98.54p on August 1. But yesterday’s national average masked a range of 17p for unleaded petrol, between the minimum (91.9p) and maximum (108.9p), equivalent to a difference in cost of £8.50 to fill a 50-litre tank.

Only 3 per cent of petrol stations — 239 out of 7,434 — listed on petrolprices.com, a cost-comparison website, were selling unleaded petrol at 91.9p.

Kay Joyce, spokewoman for the website, said: “The ranges can be ludicrous, even locally. All consumers can do is shop around.” She said that in Aberdeenshire, for example, motorists yesterday could pay anything between 91.9p and 108p. Within Greater London, the range was 91.9p to 101.9p.

A spokeswoman for Asda said: “We don’t see why you should pay more for your petrol depending on where you live. If our competitors want to charge different prices for different places that’s up to them, but we don’t think its fair.”

Roger King, the chief executive of the Road Haulage Association, said that his members were effectively excluded from any supermarket price battle because the stores’ forecourts were not equipped to deal with commercial vehicles.

Wholesale costs have declined slightly after a peak this month, which the super- markets say is the reason for their price cuts.Ruth Bridger, a petrol price analyst for the AA Motoring Trust, said: “I wonder how long it can be sustained? If supermarkets are undercutting the wholesale price, it will be very difficult for the independents to match that.”

Anne Marie Davis, an analyst for Datamonitor, said that the supermarkets could afford significantly smaller profit margins than independents because petrol was not their core business.

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