The average price of petrol in the UK has gone up to a record high of 121.61p a litre at a time when the wholesale cost has fallen, the AA said.
The all-time high has been reached while the wholesale price of petrol in north-west Europe has gone down by the equivalent of 2p a litre.
But this decrease has not been passed on to UK drivers, who have seen petrol prices rise from 109.88p a litre at the beginning of the year.
In the same period, the price of a litre of diesel has gone up from 111.52p to 123.03p.
The AA said the rise to record levels had added £5.87 to the cost of filling a typical 50-litre petrol tank and had pushed up the monthly petrol costs for a family with two cars by £24.91 - from £233.32 in January to £258.23 now.
AA president Edmund King said: "We know that record prices have squeezed profit margins for many independent retailers but would urge them to pass on whatever savings they can, partly because a fall in pump prices may bring back some of the 10-20% lost custom in recent months.
"As for the supermarkets, we hope that they will pass on savings directly to the pump and not use them to discount elsewhere."
He went on: "Realistically, the volatility of stock markets and currencies mean that we have to be prepared for wholesale prices to go back up again. But, after a week of lower wholesale prices, drivers would consider it unacceptable profiteering if at least some of the saving wasn't passed on to them."