The RAC says that current petrol prices should have dropped by around 16p per litre in recent weeks but have actually only fallen by 5.5p since hitting record levels in early July.
The latest fuel price data from Experian Catalist show that on average, supermarkets are charging 185.15p per litre for petrol - down from 190.6p on 1 July.
However, the wholesale price they pay for fuel has dropped by 19p since early June.
Last week, the AA said that forecourt prices should fall by up to 20p per litre to reflect recent wholesale price cuts.
The situation is similar for diesel, where wholesale prices have dropped 15p per litre but only 4p of this saving has been passed on to motorists.
The RAC’s fuel spokesman Simon Williams said the current situation was “one of the worst examples” of rocket and feather pricing, where forecourt costs soar as soon as wholesale prices rise but drop far slower as wholesale prices reduce.
He commented: “Following seven consecutive weeks of wholesale prices falling, we’re relieved to see the supermarkets finally reducing their prices a little. Sadly though, the UK’s biggest fuel retailers aren’t cutting their prices at the scale they should be.
“The weekly delivered wholesale cost of petrol has dropped by a huge 19p since early June, from 151p then to just 132p last week. Yet on average, supermarkets have dropped the price of a litre of unleaded by just 5.5p since the record highs of early July.”
Supermarkets have traditionally offered the lowest cost fuel, with prices usually around 4p per litre lower than the national average. However, their prices are currently less than 1p per litre lower than the average and a recent RAC study found independent sites were far more likely to be charging less than average.
Mr Williams added: “We continue to call on all major fuel retailers to go much further in reducing their prices in the coming days to ease some of the burden on drivers during what is the costliest summer ever on the roads.”