Diesel pump prices have fallen by a record 12p a litre in one month but RAC data indicates that drivers are still being overcharged.

May’s record single-month diesel price reduction was led by the big four supermarkets

Its figures for May show diesel fell from 158.91p to 146.99p by the end of the month, cutting the cost of a full 55-litre family car fill-up by £6.50.

It’s the largest cut ever seen by the RAC since it began tracking fuel prices in 2000 and means diesel is now 52p lower (£28.60 a tank) than the all-time high of 199p last summer. Meanwhile, petrol reduced by 3p from 146.35p to 143.26p – a saving of £1.70 a tank compared to the start of the month.

But amid Competition and Markets Authority concerns on supermarket fuel margins, the RAC said diesel was still 8.5p above the average charged in Northern Ireland – suggesting that the so-called “diesel overcharging scandal” isn’t over.

It’s arguing that the diesel reduction is both long overdue and smaller than it should be, given the wholesale price of diesel has been lower than petrol for 10 whole weeks.

And the motoring organisation, which has been repeatedly calling for lower prices for months, said it seemed ironic that the latest price cuts had finally come in the two weeks following the CMA’s announcement that it would be interviewing supermarket bosses about their fuel margins increasing significantly compared to four years ago.

The RAC’s data indicates that May’s record single-month diesel price reduction was led by the big four supermarkets as they brought their prices down by nearly 14p a litre (13.73p), giving an average of 142.96p (down from 156.69p). But the analysis also shows that ‘the big 4’ only shaved 2.4p off petrol (142.96p to 140.57p) in contrast to the UK average which reduced by 3p.

Asda finished May selling the cheapest diesel at 143p a litre – but this is still 4.5p more than the average across all forecourts in Northern Ireland, whether run by supermarkets or not. It also had the lowest price petrol at 139.89p – almost identical to the price being paid by drivers in Northern Ireland where supermarkets aren’t as dominant in fuel retailing.

RAC fuel spokesman Simon Williams said: “What’s happened to the price of diesel in May will no doubt give the CMA something to think about. We strongly hope the pump price reductions continue as they should. If greater transparency returns to the market, we ought to be heading for an average diesel price of 137p, similar to what drivers are paying in Northern Ireland – and a price the UK as a whole hasn’t seen since September 2021.”

In the latest fuel market development, Saudi Arabia has taken action to boost slumping oil prices by pledging further output cuts from July.

The move has already pushed up barrel prices but the RAC has said there’s no cause for panic.

Fuel spokesperson Simon Williams explained: “Diesel is still seriously overpriced due to its lower wholesale price which hasn’t been fully passed on by the biggest retailers despite a record 12p a litre fall in May so should continue to come down. And the price of petrol is also slightly too high and should fall by a couple of pence in the next week or so.

“Pump prices are now back to what they were in October 2021 with unleaded at 143.26p which means it’s very close to dropping below the previous long-term record high of 142.48p set in April 2012, but of course the Government’s 5p a litre duty discount is a big part of this. Diesel is already below its former all-time high of 147.93p from the same time.

“Oil producer group OPEC+ has taken well over three million barrels a day out of circulation so far this year, but due to the economic downturn forecourt prices have continued to fall. For that reason, we remain hopeful this won’t affect drivers for some time.”