Drivers of diesel vehicles are still paying more at the pumps than petrol users, despite wholesale prices being cheaper, the RAC says.
Throughout June, wholesale prices for diesel were 1p to 3p a litre lower than petrol, but the average cost on forecourts was 120p for a litre of diesel compared with 117p for petrol, according to the RAC's latest Fuel Watch report.
Although some retailers have closed the gap between petrol and diesel, hardly any have moved their diesel price below petrol to reflect wholesale fuel prices .
RAC fuel spokesman Simon Williams says: "The retail price of diesel has almost become a taboo subject despite 2.4 billion litres being sold in May, compared to just 1.5 billion litres of petrol.
"While there are twice as many petrol cars on the road, the increase in diesel usage shown in government statistics and the new car sales figures which show more than 50% of cars and light commercial vehicles leaving the showroom are powered by diesel, confirm that we are increasingly relying on diesel for both business and private use.
"Not only are diesel drivers now being demonised due to the increasing concerns over harmful nitrogen dioxide and particulate emissions, they are consistently having to pay a premium for their fuel."
The Fuel Watch report shows the price of diesel should, in theory, drop by about 5p a litre in the next two weeks to keep in line with lower wholesale prices. But the reality might be different.
"While retailers are obviously free to choose how much they charge for petrol and diesel, we believe that motorists deserve to be treated fairly and that means forecourt prices that reflect the wholesale market," Mr Williams adds.
In June, fuel prices remained steady thanks to a stable pound and oil prices staying flat. The average price of a litre of petrol was 117p - 14p cheaper than 12 months ago. Diesel was 15p cheaper at 120.75p.
Both have gone up since the early February average price lows of 106p for unleaded and 113p for diesel.
Simon Williams says: "Essentially, what is required is a fundamental rebalance of pricing in the retail fuel market. We need greater transparency and a fairer pricing model for both petrol and diesel."
Copyright Press Association 2015