Diesel is being sold for around 17p a litre more on average than petrol at forecourts across the UK, despite wholesale prices for both fuels being virtually identical, according to the RAC.
According to RAC Fuel Watch, the average price of a litre of petrol is 146.63p while diesel is priced at 164.26p typically.
Both fuels are selling for around 114.5p on the wholesale market, the RAC said.
RAC fuel spokesman Simon Williams said: “The forecourt price disparity between petrol and diesel across the UK is absolutely shocking given their wholesale prices are now virtually identical.
“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 added: “Considering supermarket prices are usually around 4p cheaper than the UK average, their customers should have been seeing prices under 150p weeks ago.
“We are also seeing many independent retailers charging far less than their supermarket rivals which is a sign of how much fuel retailing has changed.
“This would have been pretty unusual several years ago but is now rapidly becoming the norm.”
The RAC also highlighted membership-only chain Costco as bucking the trend, by charging just under 150p a litre for diesel.