Drivers are being charged 20p per litre more for diesel than petrol despite there being little difference in the fuels' wholesale prices.
A litre of diesel is being sold at an average of around 168p while petrol is just 148p. However, the RAC has found that diesel's wholesale price - the amount that retailers pay for fuel - was just 6p per litre more than petrol last week.
The RAC is now urging retailers to cut diesel pump prices to "fairer levels" which reflect costs instead of "subsidising" cheaper petrol by taking a margin of 20p on every litre of diesel they sell.
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Latest Government figures show 17.6 million vehicles licensed in the UK are diesel-powered, including the vast majority of vans. That represents 43% of all vehicles on the road.
RAC spokesman Simon Williams said: "Drivers of the country's diesel vehicles 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.
"The fact membership-only retailer Costco has been able to cut 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."
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