The AA is launching a smartphone price comparison app which it claims will enable its members to find the cheapest fuel in their area.

It said the tool is needed because motorists are facing a postcode lottery with some petrol retailers failing to pass on lower wholesale prices, while others forecourts in nearby locations do.

The AA said: “Over the weekend, drivers in many towns across the UK were being charged at least £2.50-a-tank more for supermarket petrol than in towns just down the road. Such is the state of road fuel price competition in the UK, officially described at the start of the year as ‘working well’.”

In its monthly review of fuel prices it said there had been a 2p per litre (ppl) fall in the wholesale price of petrol a fortnight ago and a further 3ppl reduction last week, but UK average pump prices barely moved from the late summer high of just above 138ppl.

However it conceded that with the wholesale price of petrol falling below where it started the year, pump prices did start to come down this week. At the turn of the year, petrol averaged 132ppl at the pumps, and it said this was where it should be heading if the wholesale price reduction continued.

It also reported that on Monday the price gap between Asda and the other main supermarkets was at least 1.7p a litre for diesel, and it was half a penny higher for petrol at 2.2p.

AA president Edmund King said: “UK drivers have to accept that, with such huge volatility in the international oil and fuel markets, average pump prices aren’t going to follow every twist and turn of the wholesale prices. Indeed, our track of unleaded petrol prices shows that the industry insulated drivers against some of the late July surge.

“However, we draw the line at the blatantly unfair pump price postcode lottery. Yes, Asda’s prices are exceptionally low, with some independent retailers undercutting even that. But, in other towns last weekend, putting a fuel nozzle against the head of many of our members and forcing them to pay 5p a litre more for the cheapest petrol was inexcusable.”

King added: “For that reason, we are offering AA personal members who have been with us at least a year a free smartphone fuel price comparison tool. Each time they think of filling up, they can check the cheapest pump prices nearby, check again in neighbouring areas they may be passing through and choose where to make the savings.

“If it inspires drivers to fill up away from over-priced towns, perhaps it will incentivise supermarkets and other retailers to be more competitive and charge reasonable prices more evenly across the UK. One in 50 of our younger and middle-aged AA members have had to take out a payday loan to pay for fuel in the past 18 months – the status quo is not acceptable.”