George Osborne has told energy companies and airlines that they must pass on the benefits of falling oil and gas prices to customers. 

The Treasury has begun an internal investigation into the behaviour of companies that depend on oil and gas. The price of oil is about $50 a barrel, the lowest since the 2008 financial crisis, having dropped 50 per cent over the past 12 months. Brent crude, the global benchmark, was down $2.37 to $50.73 a barrel.

The inquiry comes amid pressure from some Tories to make a commitment not to raise fuel duty after the election. There is also pressure on the Treasury to force petrol retailers to print the amount of tax paid during every petrol purchase on receipts issued by petrol stations.

The chancellor told cabinet colleagues yesterday that families should feel the rewards of the drop in oil prices through cheaper bills and flights. He added that businesses would be “watched like hawks” to see if they were reducing prices.

A Treasury spokesman hinted at unspecified further action if energy companies failed to pass on their reduced costs to customers. This could include referral to the Competition and Markets Authority. A source pointed to supermarkets that have dropped fuel prices as evidence that cuts could be passed on to customers, calling for other industries to follow suit.

Petrol prices have dropped in line with oil prices after a delay of about two weeks, prompting praise from motoring groups such as the RAC.

Prices have also been driven down by a supermarket price war. Asda, Morrisons, Sainsbury’s and Tesco have all cut petrol and diesel prices by a further 2p a litre with immediate effect.

Motorists filling up at Asda will pay 105.7p a litre for petrol or 112.7p a litre for diesel after the supermarket announced its fourteenth price cut since the end of September. Morrisons and Sainsbury’s have cut prices seven times since the beginning of December.

Mr Osborne welcomed the move but said that more action was required. Taking to Twitter, he said: “Vital this is passed on to families at petrol pumps, through utility bills and air fares.”

The RAC has predicted that petrol prices could soon fall below £1 per litre, their lowest since 2009.

“So far it has worked with fuel prices,” the Treasury source said of the government pressure on companies to pass on their reduced costs. “Pump prices are coming down; we believe it can work with other industries.”

No 10 said the prime minister was “very keen” to see price falls passed on.

The Lib Dems will confirm this week that they are dropping Vince Cable as their economy spokesman during the election campaign. Danny Alexander will take the role after performing the job of chief secretary to the Treasury for almost five years. Dr Cable will speak on business matters.