Tory MPs are demanding an assurance from Chancellor Jeremy Hunt that he will not go ahead with a planned rise in fuel duty following a warning it would add 12p to the price of a litre of petrol or diesel.

In its latest economic forecasts, the Office for Budget Responsibility (OBR) noted that the rise – which is pencilled in for next March – would bring in a record cash increase of £5.7 billion if it goes ahead.

The OBR also noted that chancellors have repeatedly frozen the duty – even though it is supposed to rise each year in line with the RPI rate of inflation – in the face of concerted opposition.

The Treasury said a final decision on the rate would not be taken until the next budget in the spring.

In a letter to the Chancellor, Tory backbencher Jonathan Gullis warned that if he tried to go ahead with the rise it would be opposed by a “substantial number” of Conservative MPs.

“The Chancellor needs to listen to motorists, van drivers and truckers, who are already being smacked hard with cripplingly high taxation, and prove to them we actually have their backs by keeping the price at the pump down,” he said.

As well as increasing by the rate of inflation, fuel duty is due to increase by a further 5p a litre in March as a temporary cut introduced by Rishi Sunak when he was chancellor is reversed.

The OBR estimated that this mean a 23% increase in March, resulting in a 12p a litre increase in the price of petrol and diesel.