Just a fraction of the fall in wholesale diesel prices has been passed on at the pumps, analysis shows.

While the cost to forecourts of buying the fuel fell nearly 20p a litre in recent weeks, average pump prices have fallen by barely 2p.

The AA study sparked accusations that some retailers are not playing fair with drivers and could be using the Iran war, which sent pump prices spiralling, as cover to rip-off motorists.

Critics call this ‘rocket and feather’ pricing, where leaps in the wholesale price are passed on almost immediately but falls take much longer.

It came as Chancellor Rachel Reeves faced fresh calls to ditch her fuel tax raid to help hard-pressed drivers as she rakes in hundreds of millions of pounds extra in VAT thanks to higher pump prices.

The AA analysis found that since 4 April, just before a US-Iran ceasefire was announced, wholesale diesel prices have plummeted from 156p a litre to 138p on Friday.

However, average diesel pump prices have only fallen 2.31p a litre since hitting their peak on 15 April.

 While wholesale petrol prices have fallen about 4p a litre in recent weeks, average pump prices have dropped barely 1p since the 15 April peak.

The AA’s fuel guru Luke Bosdet said: ‘Diesel is the road fuel that powers haulage, deliveries, business and much of the rural community.

‘The need to bring down diesel pump prices as soon as possible is essential.’

The Iran war started on 28 February and has since cost drivers more than an extra £1.5billion in higher pump prices, according to separate analysis by the RAC Foundation.

It has increased pressure on Chancellor Rachel Reeves to scrap her 5p a litre fuel duty hike, which will kick in from this September and add another £3 to the average cost of a fill-up.

The conflict has already sent the cost of filling the average 55-litre tank in a family car surging by around £14 for petrol and nearly £30 for diesel.

While other countries have cut fuel taxes to help out their drivers, Labour has insisted it will press ahead with its fuel tax raid.

The Tories and Reform UK have urged Ms Reeves to ditch the hike as pump prices are expected to remain high for several more months as the Strait of Hormuz, blockaded after the war broke out, remains effectively closed, squeezing Western oil supplies and pushing up prices.

The Liberal Democrats have gone further by demanding a 10p a litre cut to fuel duty, which is currently 52.95p a litre.

Reform UK’s Treasury spokesman, Robert Jenrick, descended on the Treasury in a party-branded double decker bus today to call again for the fuel duty hike to be scrapped.

He said: ‘We’re here to send a very strong message to Rachel Reeves, the Chancellor: do something now to help our hard-pressed motorists.

‘Chancellors and finance ministers all over the world, from Australia to Germany to Sweden to Spain to Ireland, are acting to lighten the load.’

Pointing out that taxes account for around half of the cost of a fill-up, he added: 'Rachel Reeves at the Treasury, she’s raking it in at the moment, and we’re just asking her to do a little bit to help working people across the country.’

‘Listen to the millions of Brits who are finding life hard at the moment. Take action. Lighten the load. Cut fuel duty now.’

A Treasury spokesperson said: ‘Motorists are paying more because of the war in Iran. This is not our war and that is why we did not join it.

‘We are determined to keep costs down for motorists. That’s why we have extended the 5p fuel duty cut twice until September and will continue to monitor the situation.’