Petrol prices from Tesco PLC (LSE:LON:TSCO), J Sainsbury (OTC:JSAIY) PLC (LSE:SBRY), Wm Morrison Supermarkets and Asda are not dropping in line with falling wholesale costs, despite stories that they are engaged in a price war.
Research by the RAC criticised the 'big four' supermarkets for failing to lower their prices in line with market prices for oil.
The roadside recovery company says drivers are currently paying an average of 183p per litre at pumps, which means some motorists may be paying well over £100 a tank.
"July has been an unnecessarily tough month for drivers due to the big four supermarkets’ unwillingness to cut their prices to a more a reasonable level, reflecting the consistent and significant reductions in the wholesale cost of petrol and diesel," said RAC fuel spokesman Simon Williams.
"As it was, we saw independent retailers leading the charge with fairer pump prices appearing all around the country which eventually forced the supermarkets to finally implement a more substantial cut," on July 29.
"What ought to have happened is that the biggest retailers cut their prices more significantly on a daily basis, given the wholesale price of petrol has fallen steadily over the last eight weeks," said Williams.
"Instead, average retailer margin for petrol across the industry has been up around 20p a litre for the last two weeks - more than three times its long-term average."
While several retailers decreased their prices last week, the cuts were not enough to keep up with the wholesale price of unleaded, says RAC, which believes that drivers are continuing to pay dearly for the big retailers' failure to reduce petrol pump prices in line with the price drop in wholesale unleaded.
The wholesale price of petrol meanwhile has fallen by 20p since early June, but retailers continued to raise their prices in June and only dropped their pump prices by 9p on average during July, according to the firm.
Wholesale unleaded prices are now back to their early May level of 131.75p per litre, resulting in average pump prices of around 167p per litre, but drivers are still paying an average of 183p per liter at pumps across the UK, says RAC.
For diesel, the RAC said drivers should now pay about 182p per liter, nearly £6 a tank less than the average price of 192p per liter at the end of July.
Over the past month, Brent Crude Oil was going for between US$100 and US$100 (£82-£90) a barrel on the futures market. A barrel of oil contains just under 160 litres, equating to oil costing US$0.63 to US$0.69 per litre or 52p to 57p.
Crude oil prices started the year just under US80 per barrel but since Russia's invasion of Ukraine have leapt above US$120 in March and again in June.
"The best advice for filling up is no longer to assume the supermarkets are the cheapest, but to shop around as it’s highly likely you’ll find an independent retailer which is doing the right thing and fairly reflecting their lower wholesale costs by charging a lower price," the RAC added.