Asda has cut the price of diesel to 97.7 pence per litre (ppl) amid tumbling oil prices.
The supermarket said the 2ppl reduction has taken diesel to its lowest price in over six years.
Unleaded remains at 99.7ppl.
Asda said these are the maximum prices motorists will pay at all of its 279 filling stations across the UK.
The supermarket's senior petrol director, Andy Peake, said: "We're delighted to be the first retailer to take diesel to its lowest price level in over six years.
"This latest announcement shows that we're committed to being the driving force behind lowering fuel prices across the UK regardless of where you live."
Oil prices have fallen by 30% since early December, with Brent crude sinking to less than 27 US dollars a barrel earlier this week.
Fuel price campaigners believe motorists are being denied even larger savings on diesel because of increased profits in the supply chain.
A study by FairFuelUK found that, since May last year, oil prices in sterling have fallen by more than 54% but retailers have cut pump prices by just 35%.
The report claimed that retailers' profit margin for diesel is 11ppl compared with just 5.8ppl for petrol.
Diesel should be at least 3ppl lower than petrol, the organisation added.
Campaigner Quentin Willson said there has been "weeks and weeks of increased profits on every litre of diesel sold and weeks and weeks of not passing savings on to motoring consumers".
Steve Gooding, director of motoring research charity the RAC foundation, said Asda's announcement was "a welcome move but do not mistake it for a charitable act".
He went on: "The wholesale price of diesel has been below that of petrol since early December so it is right that the benefits retailers have enjoyed in reduced costs are finally passed on to motorists.
"It is also worth remembering that by far the biggest proportion of the pump price is tax. Some 75% of what we pay on the forecourts goes to the Chancellor."