Fuel retailers have been accused of refusing to pass on Chancellor Rishi Sunak’s duty cut in full.
Mr Sunak used his spring statement on March 23 to reduce fuel duty by 5p per litre.
That was worth a saving of 6p per litre owing to the impact on VAT.
Drivers will be disappointed that prices haven’t come down further
Rod Dennis, RAC
But the average price of a litre of petrol at UK forecourts has fallen by just 3.7p per litre.
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Figures from data firm Experian Catalist show the average price per litre on Sunday was 163.6p, compared with 167.3p on March 22.
The average price of diesel fell by 2.4p per litre over the same period, from 179.7p to 177.3p.
AA fuel price spokesman Luke Bosdet said: “Why are drivers not surprised that on average a third of the petrol saving has yet to be passed on at the pumps?
“The fuel trade always disputes the accusation that pump prices shoot up like a rocket and fall like a feather. Now we know the truth.”
Mr Bosdet said the Treasury “must have expected more from UK forecourts”.
RAC spokesman Rod Dennis said: “Drivers will be disappointed that prices haven’t come down further since last week’s fuel duty cut.
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“As duty is charged on the wholesale cost of fuel, it’s the case that some retailers will be waiting for new deliveries in order to buy fuel in at the cheaper rate – meaning drivers will have to wait to see the benefit at the pumps.
“What happens to the oil price, and in turn wholesale costs, in the coming days will be crucial. If prices rise, there’s a risk the fuel duty cut will be cancelled out as retailers face higher costs.”