MOTORISTS in Plymouth pay an average of 3p less a litre for fuel than those in Exeter because the naval city has an Asda supermarket, the Echo can reveal.

Previously disparities between fuel prices in the region could be attributed to distance from oil processing plants and seasonal variations.

Now prices are dictated by Asda's pricing policy, with petrol stations near one of the supermarkets forced to reduce prices to remain competitive.

Without an Asda, petrol station owners are able to make more on their fuel.

Yesterday Morrisons, in Prince Charles Road, Beacon Heath, was selling the cheapest unleaded, at 114.9p a litre.

Yet the same supermarket was selling the fuel for 2p less in Yeovil and Plymouth.

Exeter's Pinhoe Sainsbury's was selling unleaded for 114.9p, while at the Alphington Road branch it was 115.9p. Sainsbury's stores in Plymouth and Yeovil were selling it at 112.9p.

The average price for unleaded was in Plymouth was 114.2p, in Exeter it was 3p more, at 117.5.

Industry analysts say the reason for the lower prices in Yeovil and Plymouth is because both have an Asda, which was selling unleaded at 112.9p.

The supermarket chain — owned by Walmart — introduced a low-price guarantee this year which sets a maximum price store bosses can charge for fuel, but allows them to reduce it to match the lowest local price.

AA fuel price analyst Luke Bosdet said the presence of an Asda was increasingly important in driving down prices, and he was aware of drivers in Devon unhappy with the discrepancies.

"Having a Morrisons can be helpful but prices are not guaranteed to be the lowest," he said. "Asda has a national pricing policy — either it has a set price or a cap — but if they have a competitor charging less they can go below that price.

"What we have also seen is the other supermarkets can be very slow to react to a reduction in the wholesale price of oil."

He said the AA used to see an increase in the prices charged in the Westcountry during the summer, but this now appeared to be a thing of the past.

Mr Bosdet said he had been in contact with an AA member who was incensed by the price of fuel at the M5's junction 30 services. The driver said the Esso was charging 10p a litre more than he was charged by his local garage in Hampshire.

"There are still huge discrepancies and it is something people rightly feel strongly about," he said.

Spokesmen for Morrisons and Sainsbury's said their pricing was based on a response to local market conditions.

An Esso spokesman said there were different types of Esso, only some of which were entirely owned and operated by the company.