Motorists with diesel vehicles are being 'taken for a ride' by retailers who are charging too much, the RAC has said.
The motorists' organisation said that even though the wholesale price of diesel has gone down over the last month, the price at the pump has risen.
The RAC is now calling for a price cut on diesel of 4 pence a litre.
However, petrol retailers said the RAC was wrong - and that profit margins on diesel sales were mostly tiny.
Over the last month, the gap between wholesale petrol and diesel prices - in other words what the retailers pay - has narrowed to 0.97p.
Yet motorists are still paying 6p a litre more for diesel than they are for petrol, according to the RAC.
The RAC said that companies - which tend to rely more on diesel vehicles - were being charged too much.
"It's hard not to think that business is being taken for a ride by the fuel retailers," said Simon Williams, the RAC's fuel spokesman.
"Retailers have maintained a higher margin on diesel, perhaps to subsidise petrol sales," he said.
But the industry said that the RAC had misunderstood how profits are made.
"We are extremely disappointed that the RAC made a number of assumptions that are palpably erroneous," said Brian Madderson, chairman of the Petrol Retailers Association, which represents independent forecourts.
He said that most business users who buy diesel use discount cards.
That results in profit margins for retailers of between just 0.8p and 1.3p a litre, less than they need to stay in business.
"We have to bear in mind that 900 independent sites closed between 2009 and 2013," he told the BBC.
He said that margins on petrol sales had also collapsed "by 50%" in the last two months.