Diesel drivers are still overpaying on fuel by as much as 20p per litre according to the latest RAC pump price report.
It comes as a separate report by consumer watchdogs at the Competition and Markets Authority (CMA) suggested supermarkets may be to blame for recent high fuel costs.
They found last year's supermarket pump prices appeared to be around five pence per litre more expensive than they would have been at 2019 percentage margin levels.
The CMA has suggested higher prices which drivers are paying at the pumps appear in part to reflect some weakening of competition
However, there is some good news to be had for petrol motorists, as the average price of petrol has fallen below 145p a litre for the first time in 18 months, according to RAC data.
RAC fuel spokesman Simon Williams said: "While it’s good news diesel has also dropped below 155p a litre for this first time since the end of February last year, drivers of the UK’s 12m diesel cars and countless businesses who rely it to fuel their vehicles, should be paying 20p a litre less as its wholesale price is now 4p lower than petrol's.
"This is being demonstrated very powerfully by one independent retailer in Shropshire who is currently charging 131.9p – more than 22p below the UK average. We hope this finally embarrasses the country’s biggest retailers to cut their pump prices significantly."
The expert adds that with wholesale prices of both petrol and diesel at 110p and 105p respectively, drivers should be paying no more than 142p and 137p.
As for the CMA report, the organisation now plans to conduct formal interviews with the supermarkets' senior management in order to get to the heart of the issues.
They will issue a final report no later than July 7 this year, covering the full range of issues we have considered in this market and setting out any further action that we think is needed.
Sarah Cardell, Chief Executive of the CMA, said: "The rising cost of living is putting people and businesses under sustained financial pressure. The CMA is determined to do what it can to ensure competition helps contain these pressures as much as possible.
"Our Road Fuel market study is nearly complete. Although much of the pressure on pump prices is down to global factors including Russia’s invasion of Ukraine, we have found evidence that suggests weakening retail competition is contributing to higher prices for drivers at the pumps.
"We are also concerned about the sustained higher margins on diesel compared to petrol we have seen this year.
"We are not satisfied that all the supermarkets have been sufficiently forthcoming with the evidence they have provided in our Road Fuel market study, so we will be calling them in for formal interviews to get to the bottom of what is going on."
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