The RAC said the average price of a liter of petrol increased by 3p in April alone, with the forecast cost per liter now standing at 150.0p.
Meanwhile, diesel drivers were warned they were being “seriously overcharged for diesel”, with average prices increasing by 2p per liter in April to 157.8p.
Rising pump prices since the start of the year have added around £5.50 to the cost of filling a typical 55-litre family car.
The RAC is calling on the Competition and Markets Authority (CMA) – an independent non-ministerial Government department – to address “glaring issues” with fuel retailing.
It wants the regulator – which is responsible for monitoring prices and will oversee the incoming PumpWatch price transparency scheme – to tackle “unfair retailer margins which lead to drivers getting a raw deal”.
The RAC is calling on the biggest fuel retailers to charge “fairer margins” to benefit drivers.
Doing so, they said, would end the “postcode lottery” which means some companies charge “wildly different prices” across different locations.
It would also stop so-called rocket and feather pricing, where pump prices arise when wholesale costs rise, but fall slowly when wholesale costs decline.
And it would reduce fuel prices in Britain to levels in Northern Ireland, where they are consistently 5p per liter cheaper.
Despite the CMA expressing concern about retailer margins earlier this year, the average margins for a liter of petrol and diesel is 9.5p and 17.5p respectively, according to the RAC.
The long-term average margin for both fuels is around 8.0p.
RAC fuel spokesperson Simon Williams said: “Drivers are once again having to dig deep just to go about their daily lives.
“Our data shows petrol and diesel have now gone up 10p a liter so far this year on the back of further increases in April of 3p and 2p respectively.
“Some of this is down to the oil price and the pound-to-dollar exchange rate making wholesale petroleum more expensive for retailers to buy.
“But unfortunately, it’s also very apparent that retailers are making massive margins on diesel.
“Worryingly, the CMA’s warning shot about higher retailer margins at the end of March appears to have fallen on deaf ears, meaning drivers are once again being seriously overcharged for diesel.”
