The supermarket chain dropped the price of diesel to 99.9 pence a litre on Monday and has today further reduced it to 99.7 pence a litre.
Monday’s price drop was the cheapest diesel has been since 2009.
In early December the supermarket dropped the price of petrol, which is typically cheaper than diesel, to below £1, the first time petrol had dropped below a pound a litre since 2009.
Unleaded remains at 99.9 pence a litre at the Watton garage.
Other supermarkets and some local independent retailers also responded by dropping petrol to below a pound a litre.
The RAC has called the cut in the price of diesel "overdue". The cut reflects further falls in the oil price, which is its lowest for 11 years.