Motorists are being denied a further 10p cut in petrol prices because retailers have hiked their profit margins, according to the RAC.
The motoring organisation said a further oil price fall in September pushed down the average price at forecourts nearly 7p to 162.9p.
It was the sixth biggest monthly drop since 2000, but the RAC says it should have been bigger.
“Drivers really should have seen a far bigger drop as the wholesale price of delivered petrol was around 120p for the whole month,” said RAC fuel spokesman Simon Williams.
“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 usually charge about 3.5p per litre less than the UK average but are now only about 1.5p cheaper, the RAC added.
It advised drivers to shop around for the best fuel deals rather than assume supermarkets are always the cheapest.
Diesel’s average price in September dropped 3.5p to 180.2p.
Current prices are still significantly less than the summer when a record monthly rise pushed average petrol prices over 190p per litre and diesel nudged 200p.