The average price of both petrol and diesel fell for the third successive month in January, according to the RAC.
The motoring organisation says the average cost of a litre of petrol came down by 1.32p to 120.92p, while diesel dropped by 1.27p to 130.01p.
This means filling a 55-litre fuel tank with petrol is now £6 cheaper than it was at the end of October 2018, while for a diesel car, the cost is down by £3.78.
However, the RAC says while the fall in prices is welcome, wholesale fuel costs meant retailers could have passed on greater savings.
RAC spokesperson Simon Williams said: “While it’s obviously good news the price of fuel has fallen for three months in a row, the story behind the simple forecourt average figures is quite disturbing.
“Looking at the wholesale data over the last two months reveals that we should have been paying far less for our petrol and diesel than we have been.
“Unfortunately, three of supermarket fuel retailers appear to have changed their pricing policies for the long term by increasing the margin they take on a litre of petrol to about 2p.
“This has meant the average price of unleaded has not reduced by as much as it should have, because smaller retailers nearby haven’t had to lower their prices as much in order to compete.”
Williams said that while the RAC understood retailers were free to charge what they liked for fuel, it was difficult for drivers to know whether they were being charged a fair price.
He continued: “That’s why we always encourage drivers to check the current average price of petrol and diesel, and see if they can beat that price when paying for fuel locally.
“Combine this with the fact some bigger retailers vary their prices from location to location, fuel is an essential purchase for most people and that shopping around is not that easy, and you have the perfect recipe for drivers being charged over the odds for what is essentially a commodity.”