More than 2 million people will be made worse off by government plans to increase the car tax on environmentally unfriendly vehicles bought over the last seven years, the Tories claimed yesterday.

In an attempt to force a rethink, the Conservatives warned that the changes would hit twice the number of people that have been left worse off by the abolition of the 10p starting rate of tax. The Tories will today table an amendment to the finance bill that would prevent the rise in car tax, due to be introduced in April, from being applied retrospectively to vehicles bought since 2001.

The government, which responded to growing anger among motorists in May by indicating it would defer a rise in fuel duty in September, is indicating it has no plans to withdraw the increase in vehicle excise duty.

The change is designed to encourage people to buy low-emission cars, but it has been criticised by the Tories for breaking one of the main rules of environmental taxation: that taxes should encourage good behaviour, rather than punish past decisions. The Tories said the government should act now because 2.3 million people - more than double the 1.1m households which are still losing out from the abolition of the 10p tax rate - would pay between £100 and £245 extra.