Drivers are being denied a further 10p cut in petrol prices due to major retailers hiking profit margins, according to new analysis.
The RAC said the average price of a litre of fuel in the UK fell by nearly 7p to 162.9p in September as oil prices plummeted.
This was the sixth biggest monthly drop in average petrol prices since 2000 but the motoring services company claimed the cut should have been deeper.
RAC fuel spokesman Simon Williams said: “Drivers really should have seen a far bigger drop as the wholesale price of delivered petrol was around 120p for the whole month.
“This means forecourts across the country should have been displaying prices around 152p given the long-term margin on unleaded is 7p a litre.
“In stark contrast to this, RAC Fuel Watch data has shown margins to be around 17p a litre – a huge 10p more than normal.”
Supermarkets normally charge around 3.5p per litre less than the UK average but are currently only around 1.5p cheaper.
Mr Williams noted that Morrisons is offering discounted fuel for customers who spend a certain amount of money in store.
This is a type of promotion that “tends only to be seen when supermarkets are benefitting from lower wholesale prices”, he explained.
He urged drivers to “shop around for the best deals” rather than “simply assuming” supermarkets are the cheapest fuel retailers because they have been in the past.
The average price of a litre of diesel fell by 3.5p to 180.2p last month.