Plans to raise vehicle excise duty for the most polluting cars will not cut carbon dioxide emissions as much as hoped, a cross-party committee of MPs will say today.

The House of Commons Environmental Audit Committee has said that the proposed changes have been poorly explained and communicated, and the projected carbon savings from the plans were “less than they could be”.

The Treasury should pay more attention to communicating the details and objectives of the duty and other environmental taxes in the future, and should examine the case for a more ambitious reform of vehicle excise duty (VED), the report added.

The committee was looking at the controversial tax following the announcement in this year’s Budget that VED rates were to rise for existing cars with higher emissions registered since 2001.

The committee said that attention had since focused on the 1.1 million high-carbon cars, registered between 2001 and 2006, that will have their VED more than doubled, from £210 to £430 or more.

The Government began varying road tax rates in 2001, but did not introduce the band G rate for cars that emit more than 225g/km until March 2006. In the Budget that year, Gordon Brown, then the Chancellor, said that no car registered before March 23, 2006, would have to pay the higher band G rate.

However, The Times revealed in April that when new bands were introduced by the Treasury this March it also abolished the exemption for these older cars that emitted the higher amounts of carbon dioxide from the highest rates of vehicle excise duty.

Motoring groups and other organisations said that it was unfair to penalise families who bought cars several years ago, when people knew less about the consequences of CO2 emissions.

In an interview with The Times, Alistair Darling, the Chancellor, made clear that the rise in vehicle excise duty was still under review. “It is important that there’s a clear environmental signal, but we have to be mindful of the fact that we are all taxpayers, we are all motorists.”

The MPs said that they strongly supported the Treasury’s use of VED as an environmental tax and welcomed the changes announced in the Budget.

The report said: “We are surprised that the Treasury has risked provoking political opposition for an environmental measure which, according to its projections, is of limited benefit.”

The committee was disappointed “that the Treasury had not calculated what the impacts of the [VED] Budget [changes] will be on emissions from secondhand cars, when this was one of the main objectives of the changes”.

MPs said that a failure to advertise green tax details to the public “breeds suspicion about their objectives, increasing the perception of them as revenue-raising measures with no environmental purpose”.

MPs also said that the Treasury should consider a “car scrappage scheme” to offer the drivers of high-emission cars a payment to trade in their vehicles for more efficient ones.

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