MPs have said that changes to car tax rates do not go far enough to reduce carbon emissions.
The Commons environmental audit committee said on Monday that plans to reform vehicle excise duty would have a limited impact on cutting emissions.
The MPs also claimed that the proposals had been "poorly explained and communicated", with carbon savings "far less than they could be".
"We are surprised that the Treasury has risked provoking such political opposition for an environmental measure which, according to its projections, is of limited benefit," the report said.
It also expressed disappointment that ministers had not calculated how the changes would affect emissions from second-hand cars "when this was one of the main objectives of the changes".
While it welcomed the use of VED as an environmental tax, it raised concerns that "even the projected differentials between VED bands remain too small to be effective".
"We also believe that both the proposed changes in VED rates and the objectives of VED as an environmental tax, have been poorly explained and communicated," it said.
Chairman Tim Yeo said: "The changes to car tax announced in the Budget are a step in the right direction.
"Raising the rates on high-emissions cars that are already on the road could encourage sales of more efficient models in the second-hand market.
"Meanwhile, the first-year rates being introduced for new cars will create a kind of 'showroom tax' that could be used to influence buyers of new cars to choose the most efficient model in each class."
He went on: "However, the differentials between high and low carbon cars are still nothing like wide enough to make a big impact in practice.
"According to the government's own figures, these changes will only have a very limited impact on the environment.
"The Treasury must be more ambitious, matching increased charges on high-carbon cars with discounts or rebates on low-emissions vehicles.
"The Treasury must also urgently work to ensure these changes are not unfair to vulnerable groups."
Mr Yeo added: "In all, the Treasury must do far better in communicating the details and benefits of its green tax policies.
"Making revenue neutral-tax changes, or explicitly hypothecating some of the revenue raised from green taxes towards spending on the environment, would be more effective environmentally and more acceptable to the public.
"A failure to advertise green tax details to the public, or explain them in a timely manner to Parliament, breeds suspicion about their objectives, increasing the perception of them as revenue-raising measures with no environmental purpose."
Meanwhile, three members of committee produced a minority report recommending that the vehicle excise duty increases should be reversed.
The reversal should stay "until the financial and environmental impacts are properly assessed and a car scrappage scheme has been introduced".
The minority report from Conservative committee member Graham Stuart and Liberal Democrats Jo Swinson and Martin Horwood warned that the move would be ineffective and would give green taxes a bad name.
It also accused the Treasury of failing to properly analyse how the changes would affect different income groups.
