OXFORD East MP Andrew Smith has backed Labour rebels in their bid to get the Government to overturn its proposed car tax increases.

Mr Smith, a Labour backbencher, has labelled changes which could see owners of some of the most polluting cars face a 50 per cent tax rise as unfair.

The changes mean from next year owners of older cars, which produce more carbon dioxide per kilometre, will see their car tax bills rise from £170 to £260 a year.

More than 30 Labour backbenchers have so far signed a House of Commons motion urging ministers to rethink the vehicle excise duty (VED) proposals which have been attacked by Greenpeace as "giving green taxes a bad name".

The latest tax controversy comes just weeks after Gordon Brown's Government was forced to bow to public pressure and offer money back to millions of workers who lost out from the removal of the 10p tax band.

The proposals are being labelled a retrospective tax as they apply to car owners who bought vehicles from 2001 onwards.

Mr Smith, who is currently on holiday in France, said: "I think that the Government should think again about the tax change because it's tough enough for people with the cost of petrol.

"Introducing something which is effectively a retrospective tax doesn't seem fair to people especially in the current economic climate.

"I hope and expect with the strength of public opinion and the strength of backbencher opinion that Alistair Darling will have to have a rethink on his plans.

"It would be very helpful if the Government gave a strong signal on this and on fuel duty by postponing the 2p rise."

He said he would make a decision about signing the motion when he returned next week.

The Government is set to impose a further 2p on the price of fuel in October, bringing the total fuel duty on a litre to 52.35p.

Hundreds of hauliers converged on London on Tuesday to protest at fuel prices.

The Conservative MP for Wantage and Didcot, Ed Vaizey, said: "This has caused some political difficulty for the Government. There are strong arguments against this retrospective tax.

"It's being called a poll tax on wheels because it hits the poor harder than it hits the rich."

The Lib Dem MP for Oxford West and Abingdon Dr Evan Harris said: "I do not believe that the increase in VED for polluting vehicles should be retrospective because the idea of green taxes is to change behaviour and affect future choices rather than to penalise for choices and decisions already made.

"Some constituents have written expressing their concern about retrospection."