The Treasury yesterday faced accusations that it had slipped a tax change into the Budget that would hit thousands of family car-owners.

However, it confirmed that many cars up to seven years old would be subject to a wider range of differential duty rates being brought in over the next two years aimed at driving down emissions.

It was believed that older vehicles would be exempted from the extended rates. But under the terms of Alistair Darling's first Budget, owners of larger cars bought since March 2001, would be hit.

Tory Treasury spokeswoman Justine Greening said: "This is duplicity from the Treasury who deliberately failed to make any mention of this tax grab at the time of the Budget.

"Last year, Gordon Brown promised not to backdate the CO2 charge on family cars to before 2006 and now he's broken yet another promise by backdating it to 2001.

"This hits tens of thousands of drivers and the worst hit are low-income families who simply can't afford to change cars regularly. These are people driving family cars, often bought second-hand, with 80,000 miles on the clock."

Mr Darling said the changes would come into effect from next April, and raise an additional £1.2bn in revenue by March 2011.

Paul Watters, head of roads policy at the AA, said: "The government presented the changes as means of influencing people's purchasing decisions, but it turns out that they are also penalising hard-pressed families who have been running the same car for many years."

Under the new road tax system, there will be 13 bands, with vehicles being classed based on their carbon emissions. New cars will also be subject to a so-called "showroom rate".

Owners of vehicles that emit more than 225g of carbon dioxide per kilometre will pay £430 in duty in 2010, compared with £210 this year.

The rise will be staggered, moving to £300 next year before reaching the top rate.

Medium-sized cars that emit more than 180g/km are expected to incur increases of up to £100 over the same period.

The possibility of poorer families with older cars being penalised renewed calls for a scrappage scheme, in which an allowance is paid for scrapping cars that do not meet up-to-date emission standards.

Douglas Robertson, chief executive of the Scottish Motor Trade Association, said: "Obviously it would be better for the trade if people were scrapping their old cars and buying replacements, but it is better for the environment too.

"We would like to see a scrappage scheme to get the old ones off the road, and make it easier for less well-off families to replace them. A new Jaguar produces a tenth of the emissions of a 10-year-old Fiesta, but you don't have to buy a Jaguar - just buy a new Fiesta."

Some schemes, such as the one that ran in Ireland, offered £1000 relief on registration tax for a new car, but Norway offers a straight cash allowance.

Duncan McLaren, chief executive of Friends of the Earth Scotland, said: "There will be a relatively small number of people with older vehicles who don't feel able to afford to trade them in, and a scrappage scheme would help them."