The RAC says petrol prices should have fallen by "at least" 2p per litre.
The assertion comes after the organisation's data revealed a 0.27p drop in the price of the average litre of petrol last month, while the average price of diesel fell by 0.38p per litre. As a result, the end of August saw the average litre of diesel cost 131.66p, while the average litre of petrol cost 128.88p.
However, the RAC says the prices should have fallen more dramatically after the wholesale price of petrol - the amount the fuel costs the retailers - fell by 4.38p. It was a similar story for diesel, which saw wholesale prices drop by 1p per litre.
The motoring organisation says that reduction "should" have led the average forecourt petrol price to fall by "at least" 2p per litre last month. Such a cut in prices would have meant filling a typical family car's 55-litre fuel tank would have been £1.10 cheaper at the end of August, whereas in fact the price of a fill-up fell by just 15p.
“Drivers have the right to feel angry that the price of fuel did not fall more in August than it did," said RAC fuel spokesperson Simon Williams. "With nearly 4.5p coming off the wholesale price of petrol drivers should have seen, at the very least, 2p a litre being knocked off at the pumps by the end of the month.
“While the average price charged by the supermarkets came down a little more than the UK average, they should really have led the way with larger cuts which would have spurred other retailers to reduce their prices too.
“By our calculations retailers ought to be charging around 126p for a litre of unleaded based on a wholesale price of 98p a litre, which many retailers will have bought at a couple of weeks ago, and then factoring in delivery, a reasonable margin of 5p a litre and VAT. As for the supermarkets, they could easily be selling at around 122p.
“There was a time when a 4.5p reduction in the wholesale price would have led the supermarkets to cut their prices significantly, but unfortunately those days seem to have passed as they no longer appear to have the appetite for them despite the clear wholesale market dip. Perhaps they are hedging their bets thinking there could be a further drop in the value of sterling which will could cause wholesale prices to increase again.”