The average price of diesel has fallen below 113p per litre for the first time since January 2010, according to the information group Experian Catalist.
In the last week, supermarkets have cut prices by between 2p and 4p per litre.
Last month, diesel became, on average, cheaper than petrol for the first time since July 2001.
Petrol has also fallen in price this week, hitting 115.24p per litre on average, its lowest level for three months.
Lower fuel prices have followed the falls in the price of oil.This month, the price of a barrel of Brent Crude oil fell below $50 (£32) for the first time since January this year.
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