Increased supermarket profit margins led to motorists paying an extra 6p per litre for fuel last year, a watchdog has said.
The Competition and Markets Authority (CMA) said despite supermarkets usually being the cheapest places to buy fuel, a lack of competition had caused prices to surge.
The inquiry also found that an increase in margins on diesel across all fuel retailers cost drivers an extra 13p per litre during the first five months of this year.
The regulator’s chief, Sarah Cardell said competition was “not working as well as it should be and something needs to change swiftly”.
The CMA recommends that the Government launches a “fuel finder scheme” to help drivers find better prices online.
This would allow drivers to compare pump prices at different sites through apps or satnavs.
The investigation found that Asda’s targeted fuel margins for 2023 were more than three times what they had been for 2019, while Morrisons’ target doubled in the same period.
Asda was fined £60,000 by the CMA for failing to provide relevant information in a timely manner.
Cardell said: “Competition at the pump is not working as well as it should be and something needs to change swiftly to address this.
“Drivers buying fuel at supermarkets in 2022 have paid around 6p per litre more than they would have done otherwise due to the four major supermarkets increasing their margins.
“This will have had a greater impact on vulnerable people, particularly those in areas with less choice of fuel stations.
“We need to reignite competition among fuel retailers.”
The RAC described the CMA’s report as a “landmark”, adding that motorists appeared to have lost almost £1bn as a result of increased margins.
Spokesman Simon Williams told the BBC: “It is nothing short of astounding in a cost-of-living crisis and confirms what we’ve been saying for many years that supermarkets haven’t been treating drivers fairly at the pumps.”
On Monday, unleaded petrol costs on average 143.86p per litre while diesel costs 145.54p, according to RAC data.