Britain's supermarket giants have come in for fresh criticism for the high cost of fuel.
The RAC says the big retailers are not cutting prices by as much as they should to match the "significant" drop in the cost of wholesale fuel.
The motoring group reports that the gap between pump prices and wholesale prices was the widest in almost a decade.
The price of petrol on Tuesday was about 176p per litre, but RAC analysts suggested it should be around 162p.
The BBC says that fuel prices have hit record highs this year, but are slowly starting to fall.
The RAC said that, at the start of this week, the typical petrol price at the big four supermarkets - Tesco, Asda, Sainsbury's and Morrisons - was about 174.4p per litre. Diesel was 186p.
Meanwhile the average delivered wholesale petrol price last week was 124p, while diesel was 138p.
After factoring in VAT, fuel duty and a "generous" retailer margin of 10p per litre, the RAC said "forecourts should soon be selling unleaded for no more than 162p.
Big shift
"There appears to have been a big shift in the last few months in the behaviour of the four major supermarkets, which dominate UK fuel retailing, as they are now commonly being undercut by independent retailers that are passing on the wholesale cost savings they're benefiting from to drivers at the pumps," said Simon Williams, RAC's fuel spokesman.
"This is unheard of as the supermarkets are normally at least 3p a litre cheaper than the UK average."
The BBC says the reason supermarkets are usually cheaper than independent retailers is because they buy fuel more frequently, meaning they can react quickly when wholesale prices move up or down.
Mr Williams said that was "often demonstrated by the speed at which they pass on increases on their forecourts when wholesale prices rise".
"As the supermarkets account for so much of all the fuel sold across the country and they haven't lowered their prices as much as they should have, it means average UK prices have not come down in line with the significant drop in wholesale fuel," he added.
The gap between retail and wholesale prices, excluding VAT, is the widest since the RAC started monitoring them in 2013.