A warning has been issued to millions of UK drivers who are planning on filling up their car over the next two weeks.
Drivers could potentially see a drop in fuel prices, although it would require retailers to pass on the drop in wholesale prices.
The RAC revealed that diesel drivers are being charged an extra 20p over the owners of petrol cars, and the costs "no way reflects" wholesale prices at the moment.
According to the car and motoring organisation, diesel drivers are forking out £92 to fill the average tank, with a family petrol car setting motorists back £81.
The RAC told This is Money that diesel drivers are due a "huge pump price cut" within a fortnight.
RAC spokesman Simon Williams said: "While our data shows petrol is generally being sold at a fair price at forecourts at the moment, drivers of the country’s 12million diesel cars – as well as almost every white van driver, have every right to feel hard done by as they’re paying a huge premium for the fuel, which in no way reflects its lower wholesale cost.
"For nearly a month, the gap between wholesale petrol and diesel prices has been less than 10p a litre, and in recent days it has reduced to just 3.5p.
"Yet average diesel prices at the pumps remain stubbornly high, having fallen by only 2p since the start of February.
RAC criticised retailers who have not passed on the price noting that membership-only retailer Costco cut the average price os a litre of diesel by 4p last week.
Currently, Costco is charging diesel at 157.7p per litre which is 13p lower than the national average.
Simon added: "Costco cutting the average price of a litre of diesel by 4p last week shows what’s possible, but we badly need other fuel retailers to treat drivers of diesel vehicles fairly."
The RAC found that diesel's wholesale price, which is the amount that retailers pay for fuel, was just 6p per litre more than petrol last week.
However, the average cost of a litre of diesel in the UK at the moment is around 148p.
RAC analysis found that retailers are "subsidising" cheaper petrol by taking a margin of 20p on every litre of diesel they sell.
Simon added: "If retailers now do the right thing this should reduce significantly, saving drivers who rely on diesel a lot of money every time they fill up.”
