Fuel duty could soar by 12p a litre in the spring after Rishi Sunak failed to rule out a pump price hike.
The duty has been frozen since the Conservatives returned to power in 2010.
But Treasury planning assumes a 23% climb after the next Budget, due on March 15, raising £5.7billion.
Grilled by the Commons Liaison Committee - made up of chairmen and women of parliamentary select committees - the Prime Minister left the door open to the increase.
Tory MP Harriett Baldwin, chairwoman of the Treasury Select Committee, told him: “Baked into the Chancellor's Autumn Statement is the assumption that fuel duty is going to rise by 12 pence in the spring.
“Surely you would want to confirm today to the committee that’s not going to happen?”
The PM, a former Chancellor, told her: “Having previously had his job, I always preferred it when the Prime Minister made absolutely no comment about future tax policy, so I will very much adhere to that.”
Fuel duty is the highest levy placed on diesel and petrol, at 52.95p per litre, followed by VAT at 20%.
The Government already rakes in £26.2bn from the tax.
Pressed by Ms Baldwin whether he was “going to let the Chancellor get away with that”, Mr Sunak again refused to rule out a lift.
“I am going to let the Chancellor make his policy on fiscal decisions and announce them in the normal way,” he claimed.
“Tax decisions are those that are made by the Chancellor in fiscal statements and that’s how it should be.”
Earlier, quizzed about supplying arms to Ukraine to repel Russian invaders, Mr Sunak said efforts to ship more arms and restock UK weapons caches were being hampered because factories cannot make them quickly enough.
Britain has sent thousands of missiles, including next-generation, anti-tank light weapons, to Kyiv to bolster defences.
The PM told MPs: “We are replenishing our munitions.
“I think as the Defence Secretary has probably acknowledged, the issue is less money than it is supply chain capacity in the short term.
“That is the challenge, that is the limiting factor on our ability to get some of the support we would like to see flowing to Ukraine happening quicker.”
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