The RAC has warned that fuel retailers are failing to fully enclose the huge falls in wholesale costs, impacting diesel drivers the most.
A new analysis conducted by the Royal Automobile Club said that while wholesale diesel prices fell by 32p, pump prices only dropped by 20p per litre in the eight weeks to December 11.
In a similar time frame, petrol wholesale costs lowered by 23p a litre, but average pump prices dropped by just 18p per litre after a whole month.