Supermarket Morrisons cut the price of diesel by up to 2p per litre, and Tesco, Asda and Sainsbury's followed the move with their own price cuts.
The last time diesel was cheaper than petrol was 2001, according to analysis by Experian Catalyst.
Morrisons said lower wholesale diesel costs meant it could pass on savings.
Mark Todd, petrol director for Morrisons, said: "This is a milestone in motoring and many younger drivers won't remember the last time that diesel prices were lower than unleaded."
Pump prices are influenced by factors including the distance from oil terminals and refineries, so not all diesel prices will be lower than petrol, the supermarkets said.
In July 2001, diesel cost 77.92 pence per litre on average, compared with 77.94 pence for a litre of petrol, according to Experian Catalyst data. A month prior, diesel had been as much as a penny cheaper per litre, and a price difference of just under two pence was found in 2000.
There may be some delay until UK motorists see the price dip among all competitors. Experian Catalyst said the on Sunday UK average prices narrowed to 117.10 pence per litre for petrol and 118.33 pence for diesel.
Diesel is normally more expensive than petrol per litre for a variety of reasons, according to the United Kingdom Petroleum Industry Association.
While most recent investment in refineries UK has been for unleaded petrol production, more efficient engines mean petrol demand has been decreasing since the early 1990s, depressing prices by comparison to diesel, it says.
However, increased supply of diesel from refineries outside the UK is likely to be the reason behind this recent price flip, according to the RAC.
"With the price of crude oil standing at $56.50 and the pound relatively strong against the US dollar - the fuel trading currency - there is scope for further price cuts of up to 4p per litre off diesel over the next fortnight, and unleaded may even benefit from a slight reduction too due to the continued oversupply of oil," said RAC fuel spokesman Simon Williams.
"Drivers of unleaded petrol vehicles should not feel short changed - what we are seeing is the diesel pump price coming down to a level closer to what it should be due to the effects of greater refining capacity in the Middle East."
Wholesale prices for diesel have been cheaper than petrol since 27 May, the RAC said.
As crude oil has become cheaper, both petrol and diesel prices have fallen too. However, petrol prices have fallen more slowly allowing diesel to become cheaper, says James Challinor, research director for research and consultancy group Wood Mackenzie. This is caused in part by demand for petrol from the US.
"Fundamentally it is strong demand for European gasoline [petrol] from the US and West Africa that is supporting the wholesale" petrol price, he said.
Refinery outages in Latin America are creating demand for exports of US petrol into that region, which in turn leads European petrol to be exported to the US, he added.
The price of diesel has fallen below petrol at some supermarkets for the first time in more than a decade, after the big chains announced price cuts.