The RAC is calling for a cut of 4p a litre in the price of diesel due to a lack of parity between wholesale and pump prices.
Diesel costs around 1p more than regular unleaded petrol in the wholesale market – but it can cost up to 6p a litre more by the time it reaches forecourts.
The RAC is demanding this be corrected with the 4p cut – which would still leave retailers with some additional profit over petrol sales.
The RAC’s Simon Williams said: “It’s hard not to think that business is being taken for a ride by the fuel retailers.
“Traditionally, business runs on diesel, and with sales of diesel at an all-time high the retailers have maintained a higher margin on diesel, perhaps to subsidise petrol sales”.
In February 2015, diesel sales clocked their fifth highest total since 1990 – but it was the eight lowest month for petrol sales since 1990.
Fuel sales rose 3.5 per cent last month; diesel was up 4.5 per cent and petrol rose 2 per cent.