The battle has just heated up on the front line of the supermarkets' price war, after Morrisons announced plans to cut 2p a litre off the price of diesel, meaning the cost of diesel will fall below petrol for the first time in 14 years.
Morrisons' announcement came after retailers were criticised by motoring organisations for keeping the price of diesel artificially high, despite the fact oil prices were falling.
However, Morrisons cautioned that at some petrol stations, "local competitive factors" mean diesel prices may be higher at some of its forecourts.
According to petrolprices.com, the average price of diesel is currently 118.2p, with the lowest price in the UK at 112.9p, while the highest is 134.9p. Meanwhile, the average price of unleaded petrol is 116.75p.
While government figures have shown there are twice as many petrol as diesel cars on the road, more than half of cars and light commercial vehicles being sold are powered by diesel.
But at the beginning of July, RAC fuel spokesman Simon Williams pointed out that the retail price of diesel had "almost become a taboo subject".