'Magical thinking' on Heathrow expansion
Heathrow expansion can only be justified if the government proves it will not breach laws on climate change and pollution, MPs say.
Ministers say a third runway will not exceed environment limits.
However, the Commons Environmental Audit Committee has accused the government of "magical thinking" - wishing the problem away without a proper solution.
They say ministers must show the expansion will not fuel climate change.
Committee chair Mary Creagh told BBC News: "There's plenty of talk about how the government wants to solve environmental problems at Heathrow, but a total absence of any policy guarantees.
"The implication of this is that they think other sectors of the economy like energy and industry are going to have to cut their carbon emissions even more so people can fly more - but the government's been told by its own advisors (the Committee on Climate Change) that's not possible."
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The MPs also criticised the government's reliance on a projected increase in electric vehicles on the roads to keep local air pollution within safe limits.
"The government has missed already its targets for electric vehicles," Ms Creagh said. "Our committee has no confidence it will meet its target for 2020 or 2030. Ministers have got to put proper policies in place instead of relying on magical thinking."
The committee previously urged a step change in the way the government tackles environmental issues at Heathrow, but says there is little evidence this has happened.
The UK has already breached EU limits in London for the pollutant NO2 for 2017. The committee says a new air quality strategy is urgently needed to ensure that airport expansion does not harm public health.
The government has said after Brexit that EU environmental laws will be imported wholesale into the UK, but the MPs say they have seen no guarantees that the government will keep pace with future EU air quality laws.
The report calls on the ministers to implement an alert system for nearby residents who are especially vulnerable to short-term exposure to air pollution.
On climate change, the MPs complain that international aviation emissions from an expanded Heathrow will be 15% higher than the level previously set for 2028-32. They say the government must show how the slack will be taken up by other sectors of the economy, which are already struggling to meet their own emissions targets.
They say measures on noise lack ambition, with no precision on the timing of a night flight ban and little evidence that predictable respite can be achieved.
The report was welcomed by John Stewart, the chair of HACAN, the campaign group that opposes Heathrow expansion.
"The government and Heathrow Airport have got to up their game big-time if they are to have any chance of getting a third runway," he said.
"They have got to prove they can deliver on noise, climate and air pollution - not just say they can."
The report comes just weeks after the government launched a public consultation on a third runway, which ends on 25 May.
Later this year or early next year MPs are expected to be asked to vote on the runway.
A Department for Transport spokesman said: "We take our air quality commitments extremely seriously and have been very clear that the new runway will not get the go-ahead unless air quality requirements can be met."
The spokesman said the government has no plans to "water down" its ambitions on cutting aviation emissions and remains committed to meeting emission reductions targets under the Climate Change Act.
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