Petrol and diesel prices fell for the second consecutive month in December, according to data from the RAC.
It says the average price of petrol per litre went from 123.67p to 120.92p, while diesel went from 133.09p to 130.1p.
Despite the reduction in price, the RAC says fuel should have become much cheaper due to a sharp drop in the price of oil, which fell by 14.5%.
It says if this saving was passed on at the pumps, it would mean petrol priced at 113p per litre and diesel at 120p.
The RAC further warned that with the exception of Asda, supermarkets appeared to no longer be competing on price, meaning the market was not being driven lower.
RAC spokesperson Simon Williams said: “While it’s good news fuel prices have fallen for the second month in a row, drivers should feel cheated they have not come down further.
“The problem is twofold: firstly, there should be a cut in the price of petrol to properly reflect lower wholesale prices and secondly, three of our biggest supermarkets appear no longer to be competing on the price of unleaded, in particular, as closely with the lowest price supermarket retailer, Asda.
“The decision by supermarkets to take more profit on a litre has led to every driver having to pay more to fill up than they should have to. This is because the UK average is negatively affected as other retailers are not being forced through competition to lower their prices.”
Williams described this situation as ‘highly unusual’, and said it had not been seen before.
He added: “If this new pricing behaviour continues into 2019 this could spell a bleaker year for drivers at the pumps no matter what happens to the price of wholesale fuel.
“It is important to point out that every fuel retailer is at liberty to charge what they want for their petrol or diesel so, aside from the government taking the unlikely step of capping retailer margins, there is little that can be done to change this.
“Sadly, it means motorists are completely at the mercy of retailers because just one major supermarket passing on savings in the wholesale price isn’t enough to change prices across the majority of UK forecourts.”