Diesel prices fell by a record of nearly 12p a litre last month, figures show.
The RAC said the average price of the fuel at UK forecourts dropped from 158.9p to 147p in May.
Despite this being the largest monthly reduction since the motoring services company began tracking fuel prices in 2000, it believes the cut should have been higher to fully reflect changes in the wholesale market.
The price drop has lowered the cost of filling a typical 55-litre family car by about £6.50.
May was the seventh consecutive month in which pump prices fell.
The average price of a litre of petrol dropped by more than 3p from 146.5p to 143.3p last month.
The Competition and Markets Authority watchdog said last month that indications showed higher pump prices in 2022 “appear in part to reflect some weakening of competition”.
It said it had found evidence that weakening competition among the supermarkets had pushed up prices for drivers at the pump.
The RAC’s fuel spokesperson, Simon Williams, said: “After calling for big pump price cuts for weeks, we were pleased to see that May was the month where this finally happened. The fact it appears to have been prompted by the Competition and Markets Authority’s mid-month announcement about weakening competition in fuel retailing is surely not a coincidence.
“A 12p reduction in the price of diesel in one month is something we haven’t seen in nearly 23 years of monitoring prices. But despite this, it’s still galling to see that a litre of diesel is 8.5p cheaper in Northern Ireland than it is in the rest of the UK.
“This points to a more transparent and competitive fuel market there, something drivers in the rest of the UK would very much like to see, particularly with money being so tight in the cost of living crisis.”
