The disparity has been due to some retailers failing to pass on a dip in wholesale petrol costs, the AA said.
It added that the fall in average petrol prices in the UK appeared to have stalled.
The average price hit a year high of 140.03p a litre on March 3 and fell to 137.72p on March 24. Since then, petrol pump prices have stalled close to 137.9p a litre and averaged 137.84p on Tuesday.
The AA said average diesel prices continue to fall, down from a year high of 146.46p a litre on March 4 to 143.57p on Easter Monday. Record fuel prices were set in April last year, with petrol hitting 142.48p a litre and diesel 147.91p.
Wholesale petrol costs started to rise again last week as the market anticipated greater demand with the start of the United States' motoring season.
AA president Edmund King said: "Fuel pricing in the UK, US, and Europe over the past 12 months has been characterised by a series of severe spikes, surging petrol prices up and down by the equivalent of 10p a litre. This has had a severe impact on consumer demand.
He went on: "Unlike the US, drivers in the UK and Europe have been left high and dry by the lack of fuel price transparency. This has denied them the ability to spot short-term pump price spikes and prepare their budgets and planning for the hit.
"It has also allowed retailers to decide when and where savings for this essential part of family spending are passed on."
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