The government has not set strict enough targets for cutting carbon emissions from transport, a group of MPs has said.
The Commons environmental audit committee expressed doubt that the government "appreciates the magnitude and urgency" of cutting CO2 emissions.
It has been looking into government policy in this area and published a report on its findings on Monday.
The committee said it was concerned about the Department for Transport's reliance on CO2 projections for 2050 as evidence that its policies are on the right track.
The report calls for more investment for emerging technologies such as carbon-capture and storage, off-shore wind and cuts in council tax and stamp duty for homes built to "high environmental standards".
And in an open letter to Gordon Brown, the committee's chairman, Tim Yeo, urged the chancellor to increase environmental and transport taxes, encourage incentives to promote greater fuel and carbon efficiency in cars and to raise taxes for air travel.
Yeo said: "The Department claims that its policies are on the right track - but its own projections of future carbon emissions suggest that CO2 from transport will be no lower in 2050 than in 1990.
"This contrasts with the government's targets for the UK as a whole to make carbon cuts of at least 60 per cent from 1990 levels by mid-century.
"This is precisely the lack of ambition, the sense of entitlement owing to transport as a special case, that we highlighted throughout our report.
"We wonder whether DfT truly appreciates the magnitude and urgency of this issue, no matter the array of climate change policies it can point to."
Yeo also raised questions over the government's plans to expand airport capacity.
"On aviation, the government deserves praise for its efforts towards including the aviation sector in the EU emissions trading scheme," Yeo said.
"But we are concerned that the department has rejected our recommendation that it embark on a fundamental rethink of its airport expansion policy.
"We remain to be convinced, for instance, that the government's decision that there should "only" be two extra runways in the South East rather than three really is proof that it is has a sustainable aviation policy."
"Whether even these runways are ever built in practice is perhaps another matter," he added.
