The PRA has reacted furiously and accused the RAC of an alarming lack of research after it claimed retailers are profiteering on diesel and taking high margins because much of the diesel sold goes to business users who are less price sensitive.

RAC fuel spokesman Simon Williams said: "It’s hard not to think that business is being taken for a ride by the fuel retailers." He suggested retailers had maintained a higher margin on diesel, perhaps to subsidise petrol sales. He said the gap between the wholesale price of diesel and petrol was 1ppl but average forecourt prices were currently 6ppl apart, and he called for a 4ppl cut to diesel prices.

PRA chairman Brian Madderson said: "They haven’t done their homework at all. These comments fail to acknowledge the increasing impact of fuel cards, which are used by a significant number of businesses, and that means they are complete nonsense. This highlights the alarming lack of research by some motoring organisations."

He said that transactions made on fuel cards in the UK are now worth over 8 billion litres per annum.

And he added: "Currently the margin available on petrol is extremely low and so higher margins may be taken on diesel after adjusting for the severe margin depressing effect of fuel cards to the independent retailer sector."