Lib Dem's Danny Alexander says petrol prices at the pumps must fall Lib Dem minister wants motorists to benefit from wholesale price cuts It would be an 'outrage' if companies keep prices unnecessarily high Critics likely to say Mr Alexander should propose further cuts to fuel duty Serious Fraud Office investigating claims that oil companies fixed prices By James Chapman for the Daily Mail and Matt Chorley for MailOnline Danny Alexander is writing to fuel companies and supermarkets asking them to ensure motorists benefit from wholesale price cuts Drivers have a 'right' to expect petrol prices to fall at the pumps when the wholesale cost of oil drops, a cabinet minister will today warn suppliers. Chief Secretary to the Treasury Danny Alexander is writing to fuel companies and supermarkets asking them to ensure motorists benefit from wholesale price cuts. The Lib Dem minister will say in a speech today that it would be an 'outrage' if the suspicion that the companies keep prices unnecessarily high was proved correct. 'I will be writing to the major players involved in the distribution and sale of petrol and diesel seeking their assurance that they are doing all they can to pass on the benefit of falling oil prices as quickly as possible,' Mr Alexander will say. When the price of oil falls, the public have a right to expect pump prices to fall like a stone, not a feather. Critics are likely to say that Mr Alexander should be proposing further cuts to fuel duty - slashed and then frozen for five years by Chancellor George Osborne - rather than pleading with petrol giants. But Mr Alexander, speaking in Aberdeen, will add: 'Especially in the current economic circumstances, people would rightly be angry if they feel that pump prices don't fall as much as they should on the back of falling oil prices. 'I believe it's called the rocket and feather effect. The public have a suspicion that when the price of oil rises, pump prices go up like a rocket. But when the price of oil falls, pump prices drift down like a feather. This has been investigated before and no conclusive evidence was found. But even if there were a suspicion it could be true this time it would be an outrage. He will add: 'I have made sure over the last four years that Government has helped with the cost of fuel. And when the oil price falls, industry must do all it can to help too. In 2011, the Government cut fuel duty by 1p and subsequently froze the tax - a move that will have cost the Exchequer a massive £22.5 billion by next year's general election. The Serious Fraud Office is carrying out a review into allegations that oil companies have ripped off drivers by fixing prices. Firms have repeatedly denied that the falling price of crude in recent months has not been properly reflected at the pumps. Since June, oil has fallen by $27 a barrel and average forecourt prices have dropped by 6.75p a litre. Petrol and diesel prices will be about 20p per litre cheaper by the end of this Parliament compared with what would have been under Labour's plans for staggered hikes, according to the Treasury. Supermarkets have cut prices recently, reducing the cost of fuel at the pumps by 2p a litre. The AA called it a 'timely boost'. Asda led the way, with petrol falling to 120.7p a litre for petrol and 124.7p for diesel - its lowest prices since December 2010. Competitors then followed suit. However, the reductions were less than half the five pence per litre fall in wholesale petrol costs over the previous fortnight. When the price of oil falls, the public have a right to expect pump prices to fall like a stone, not a feather Lib Dem minister Danny Alexander Lobby group FairFuelUK called for an inquiry into petrol prices by the Office of Fair Trading and also demanded a 3p cut in fuel duty. Quentin Willson of FairFuelUK said: 'We can't have a posse of speculative operators, led by the big banks, playing games with our financial future. The recession in 2007/08 began when the supply chain buckled under the weight of the 145 dollar barrel. We simply cannot ever allow that to happen again. In our opinion, the sooner we have legislation to stop opportunistic oil speculation by non-physical trading activity, the better. RAC Foundation director Professor Stephen Glaister said: 'It is encouraging that Mr Alexander shares the concerns of the nation's drivers but in a way he is passing the buck. The biggest driver of pump prices remains the Government. Well over 60 per cent of the price is tax. And that share could rise sharply with the reintroduction of the fuel duty escalator. Though the Chancellor has said he will freeze fuel duty until the election, he has also committed to hikes in duty above the rate of inflation if the oil price falls to 75 dollars a barrel. Which is it to be? Petrol prices have fallen sharply in recent weeks, but ministers beleive they could drop further if the drop in oil costs was passed on in full AA president Edmund King welcomed the Government move, but added: 'They themselves could do more.' He went on: 'First, policies to help strengthen the pound by just 10 cents against the dollar would double the potential for a 2p-a-litre fall in the price of petrol to 4p. Well over 60 per cent of the price is tax Stephen Glaister, RAC Foundation 'Secondly, the Government's failure to introduce fuel price transparency, showing the relationship between oil, wholesale and pump prices, has helped no one. Consumers can't see if they are paying a fair price for their fuel and retailers are less able to defend themselves against claims of profiteering.' Mr King went on: 'The Treasury's Chief Secretary, as a rural Scottish MP who once took on a supermarket giant to get fairer prices in his constituency, will also understand the need to encourage fuel retailers in small rural towns of 10,000-15,000 to pass on price cuts more quickly. Lower pump prices also expose more fully the burden of fuel taxation, currently around 65 per cent of the cost of a litre of petrol. The Government has undertaken to freeze fuel duty until the election, but any political party that advocates a fuel duty increase afterwards will suffer a backlash - such has been the trauma for families and businesses of pump price spikes from 2011 through to last year.