But the Treasury denied he had agreed to reverse the move to hit gas-guzzling cars with a rise in Vehicle Excise Duty, which critics say would hit up to 18 million motorists.

The Tories will use a Commons debate on Wednesday to try to reverse the changes, which would apply retrospectively to existing vehicles.

Reports suggest a Labour revolt has been averted by Mr Darling promising to "fix" the issue.

The Treasury insisted he has "not made any concessions or promises to make any in the future" and said there are "no plans whatever" to abandon the changes.

"The Chancellor listened to people's concerns and acknowledged those concerns," a spokesman said.

Prime Minister Gordon Brown has insisted the move, which will see the most polluting cars pay £950, will actually benefit most drivers.

But Tory leader David Cameron warned him: "If you don't get rid of it they will get rid of you."