The cost of a litre of diesel in the UK is 8p more than the next-most expensive countries, Finland and Ireland, data collated by the RAC shows.
Despite being just across the English Channel, French drivers also pay 11p less per litre than their British counterparts.
The RAC said these cost differences could not just be explained by differences in international markets.
According to the motoring group, retailers have average profit margins of 14p per litre of petrol and 16p per litre of diesel.
That compares to long-term average margins of 8p for both fuels – meaning supermarkets and forecourts are now making double what they have historically made on diesel.
In 2019, before the coronavirus pandemic, the Big Four supermarkets were making a 3p margin per litre of petrol and 8p per litre of diesel.
The “persistent overcharging” at the pumps means it now costs an extra £3 to fill a typical family car with petrol and an extra £5.50 to fill one with diesel, analysis by the RAC found.
Mr Williams said the numbers underlined the need for the Government to roll out an online “Pumpwatch” scheme, which would boost competition by allowing drivers to easily compare prices and find the cheapest deals.
This has already been promised by ministers, along with a price monitoring body, after a 2022 report by the Competition and Markets Authority found that drivers had been overcharged £900m by retailers.
Mr Williams added: “Whichever party, or parties, form the next government, it’s essential they ensure the new Pumpwatch scheme, which became law just before the election, is set up as quickly as possible.”
Asda has in the past said it is “focused on providing our customers with the best value at the pumps”.
Morrisons has said it is “extremely competitive on fuel pricing and although margins have increased, they remain very low”.
Tesco has said it is “committed to providing customers with great value, competitively priced fuel and we regularly monitor fuel prices”.
Sainsbury’s chairman Martin Scicluna last year said: “To be very, very clear, we are not profiteering and we are not rip-off retailers.”