Drivers are paying 17p more per litre for diesel than petrol across the UK, despite identical wholesale prices.
The average price of a litre of petrol is 146.63p, while diesel costs an average of 164.26p at the forecourt, according to the RAC.
On the wholesale market, both fuels are selling for around 114.5p.
The price disparity was described as “absolutely shocking” by RAC fuel spokesperson Simon Williams.
He said: “At the beginning of March, wholesale diesel was only 6p more expensive than petrol yet there was a 20p-a-litre gap between both fuels on the forecourt.
“Now the two fuels are identical on the wholesale market, and there’s still more than 17p difference at the pump.”
Mr Williams said people who fill up at supermarket petrol stations should have been seeing prices at less than 150p per litre some weeks ago, as these stations are typically 4p per litre cheaper than the UK average.
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But many independent retailers are now charging far less than supermarket rivals, an indication of how much fuel retailing has changed, according to Mr Williams.
“This would have been pretty unusual several years ago but is now rapidly becoming the norm,” he said.
Membership-only chain Costco has been highlighted by RAC by charging just under 150p a litre for diesel.
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In January, petrol prices fell below 150p a litre for the first time since the Russian invasion of Ukraine. The decline was described by the AA as a “huge relief for drivers”.
Diesel did not fall at the same rate, dropping to an average of 172.2p per litre, falling another 7.9p to current costs.
