A failure by petrol firms and supermarkets to pass on the full benefit of falling oil prices to customers filling up at the pumps would be an “outrage”, a Cabinet Minister will warn.

Treasury Chief Secretary Danny Alexander is to demand guarantees from fuel companies and distributors that they are doing all they can to pass on the price cuts to hard-pressed motorists.

Mr Alexander will use a speech in Aberdeen to say consumers feel petrol prices rise “like a rocket” when oil costs go up, but fall “like a feather” when they come down.

And people would “rightly be angry” if they felt prices were not coming down as much as they should.

Brent crude slumped as low as $82 (£51) a barrel earlier this week, its lowest level in just over four years due to concerns about over-supply.

The Liberal Democrat frontbencher will say: “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.”

Mr Alexander promises to write to the industry’s major players “seeking their assurance that they are doing all they can to pass on the benefit of falling oil prices as quickly as possible”.

He 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.”

Pointing to the Treasury’s fuel duty freeze, Mr Alexander will say: “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.”