The RAC is calling on fuel retailers to reduce the price of diesel at the pump due to falling wholesale costs.
Last Friday, the wholesale cost of diesel was 104p per litre, just half a pence more than the price of unleaded. But the average retail price for diesel for motorists was 136.36p - 6p more expensive than petrol at 130.22p.
Now the RAC is urging those in the fuel industry to explain why the diesel retail price is so much greater than petrol when the wholesale prices are so similar - and to pass on the savings to motorists too.
RAC fuel spokesman Simon Williams said: "Transparent, fair fuel pricing is vital for the economy and to maintain the trust of motorists.
He added: "In the last year retailers have been noticeably quicker to pass on reductions in the wholesale price to forecourts, but we are now seeing an unfortunate blip in that trend where diesel prices are higher than they really need to be.
"There has been talk of the fuel retailers using higher diesel prices to keep petrol prices lower, but whether or not that's the case, the simplest way to operate must be to make sure retail prices always reflect wholesale prices proportionately. This has to be fairer in the long run."
However, the price of both petrol and diesel has fallen over the past year. Unleaded is now 4p a litre cheaper than it was a year ago, while diesel is nearly 3p cheaper.