Petrol is about 12 pence-per-litre too expensive, while diesel is 10p too much as retailers fail to pass on wholesale price reduction, says the RAC.
Fuel prices have been at record levels this year, which has largely been attributed to rising oil prices.
However, last week the cost of oil fell by $10 a barrel to $73.18 in response to news of the Omicron Covid variant, leading to a further drop in the wholesale price of fuel.
This should have been reflected at the pumps with the average price of unleaded dropping from 147.64p a litre to 135p, and diesel falling from 150.85p to 141p, the RAC believes.
Simon Williams, RAC fuel spokesman, said: “We estimate that retailers are now making around 19p a litre which is shocking when you consider their average margin pre-Covid was 6p.
“The profit on diesel is around 15p a litre with a similar long-term average margin to petrol.
“Based on the fact the biggest retailers buy new supply every week we believe unleaded is 12p too expensive and diesel about 10p too dear."
He added: “While retailers might resent the RAC pointing out that their fuel is overpriced, this doesn’t change the fact that they should cut.
“And if they don’t, we feel they will lose credibility with drivers, although it’s very difficult for motorists to vote with their feet because they have nowhere else to go."
