The drivers of nine million cars, nearly half of all those in Britain, will be worse off under government plans to raise vehicle duty for polluting cars, according to figures from the Treasury.

The news that so many hard-pressed drivers of older and second-hand cars will be hit has raised the spectre of another revolt by Labour MPs after the furore over the abolition of the lowest income tax band.

Projected figures released by minister Angela Eagle revealed that owners of 43 percent of Britain's 22 million cars would be worse off under the proposals, with fewer than one in five seeing their tax bill fall.

The Conservatives said the news contradicted Prime Minister Gordon Brown's comment in parliament that the majority of motorists would benefit from the changes.

Currently, cars bought after March 2001 and registered before March 2006 are liable for a maximum of 210 pounds in car tax.

Under the government's plans, from 2010 the most polluting cars would pay 455 pounds, while those with the least CO2 emissions would pay less than now.

Eagle said the move was a bid "to strengthen the environmental incentive to develop and purchase fuel-efficient cars".

But the move has provoked criticism as the changes would affect people who had already bought their cars and also include vehicles such as the popular people-carriers that are not regarded as "gas guzzlers".

When quizzed about the proposals in parliament last month, Brown told Conservative leader David Cameron that most families would not be hit by the rise.

"If the right honourable gentleman looks in detail at the proposal, he will see that the majority of drivers will benefit from it," Brown said at the time.

However, in a written parliamentary answer on Wednesday, Eagle said that out of the predicted 21.9 million cars on Britain's streets by 2010, 18 percent would be better off, 39 percent would see no change and 43 percent would see their tax rise.

Tory shadow Chancellor George Osborne said Brown should apologise to MPs for misleading parliament. He said the Labour government was sleepwalking into "another 10p tax fiasco".

"Nine million families face higher car taxes at a time when few can afford it," he told parliament.

"Poorer drivers will be penalised because the tax is retrospective and hits second-hand cars.

Chancellor Alistair Darling said the question was how the government encouraged people to use less energy and car manufacturers produce more efficient cars.

However he is likely to come under pressure from Labour MPs to ditch the plans after promising to look at them again in the Autumn.

"I think it will be sorted - I think Alistair Darling has got the message loud and clear," said Labour backbench MP Martin Salter.

"The problem is the implementation of this policy," he told BBC TV. "It's all very well saying that motorists should make green choices, of course they should make green choices.

"But surely to goodness we have got to give people, particularly on low incomes with second hand cars, longer than two years to change their vehicles and to make those green choices."