Boris Johnson looks on the brink of killing off Heathrow's third runway plan today after campaigners won a crucial legal battle.
The government has effectively washed its hands of the £14billion project after the Court of Appeal ruled that it had not yet met environmental standards.
Judges declared that the Government had failed to take account of its commitment to the Paris Agreement on climate change when setting out its support for the airport expansion in its National Policy Statement (NPS).
The Prime Minister - who as London Mayor once vowed to 'lie down in front of bulldozers' to prevent the new runway - now seems set to pull support for the scheme.
Transport Secretary Grant Shapps said the government will not back an appeal to the Supreme Court, saying any expansion will be 'industry led'.
There are claims that Mr Johnson and his aides have been looking at alternatives to the plans, including enlarging Gatwick in Sussex.
The challenge today was brought by a group of councils in London, environmental charities including Greenpeace, Friends Of The Earth and Plan B, and London Mayor Sadiq Khan.
Heathrow has said it will appeal the judgement insisting the issues raised by the court were 'eminently fixable'. It has warned that failure to go ahead with the project - which has been in preparation for more than 15 years - would destroy hopes for 'global Britain'.
However, it is unclear whether the case can proceed without government support.
Environmental campaigners celebrate the Court of Appeal's decision today to block plans for a third runway
Boris Johnson (pictured at a homeless centre today) is personally against the scheme and when he was London mayor he promised to 'lie down... in front of those bulldozers' to stop the runway being built
Lords Justice Lindblom, Singh and Haddon-Cave said the Government did not take enough account of its commitment to the Paris Agreement when supporting the runway at Heathrow (pictured)
Transport Secretary Grant Shapps said the government will not back an appeal to the Supreme Court, saying any expansion will be 'industry led'
The UN's Paris Agreement, which came into force in November 2016, commits signatories to tackling climate change by taking measures to limit global warming to well below 2C.
In a summary of the ruling, Lord Justice Lindblom told a packed court: 'The Paris Agreement ought to have been taken into account by the Secretary of State in the preparation of the NPS and an explanation given as to how it was taken into account, but it was not.'
The judge said that, having seen the decision in advance, the Government did not oppose a declaration that the NPS was unlawful and had not sought permission to appeal to the Supreme Court.
Clarifying the meaning of the ruling, Lord Justice Lindblom added: 'Our decision should be properly understood.
'We have not decided, and could not decide, that there will be no third runway at Heathrow.
'We have not found that a national policy statement supporting this project is necessarily incompatible with the United Kingdom's commitment to reducing carbon emissions and mitigating climate change under the Paris Agreement, or with any other policy the Government may adopt or international obligation it may undertake.
What happens now? How today's ruling doesn't mean Heathrow's third runway project is over
Today, judges were specifically asked to rule on whether the Government had considered its duties under the Paris Agreement on climate change when supporting the third runway.
Specifically, the justices considered the official national policy document - which set out minsters' views on the project and attempted to justify their decision to support it.
The Court of Appeal today ruled the document did not sufficiently explain how the runway could be compatible with the Paris Agreement, but this does not mean judges have decided the project cannot go ahead.
Boris Johnson has always personally opposed the runway, but if he decides to swallow his concerns and renew the Government's backing he could rewrite the policy document to include more detail about how its proposals are compatible with Paris.
He could also support Heathrow's appeal to the Supreme Court over today's ruling. Alternatively, he could use it as an excuse to abandon the Government's backing for the runway altogether.
'The consequence of our decision is that the Government will now have the opportunity to reconsider the NPS in accordance with the clear statutory requirements that Parliament has imposed.'
Lawyers for the campaigners told the court at a hearing in October, when considering the proposals, that then-transport secretary Chris Grayling did not take enough account of environmental legislation or of climate change issues.
Mr Johnson opposed the expansion of the west London airport when he was London's mayor and promised to 'lie down ... in front of those bulldozers' to stop the runway being built.
He repeated his opposition as MP for Uxbridge in west London, which would be affected by the plans, and earlier this month said there was 'no immediate prospect' of construction beginning.
Mr Shapps said today: 'Airport expansion is core to boosting global connectivity. We also take seriously our commitment to the environment.
'This Govt won't appeal today's judgement given our manifesto makes clear any #Heathrowexpansion will be industry led.'
A Heathrow spokeswoman said: 'The Court of Appeal dismissed all appeals against the Government - including on noise and air quality - apart from one which is eminently fixable.
'We will appeal to the Supreme Court on this one issue and are confident that we will be successful.
'In the meantime, we are ready to work with the Government to fix the issue that the court has raised.
'Heathrow has taken a lead in getting the UK aviation sector to commit to a plan to get to net zero emissions by 2050, in line with the Paris Accord.
'Expanding Heathrow, Britain's biggest port and only hub, is essential to achieving the Prime Minister's vision of global Britain.
'We will get it done the right way, without jeopardising the planet's future. Let's get Heathrow done.'
The coalition of councils, environmental campaigners and Mr Khan appealed against a High Court ruling in May, which rejected four separate judicial reviews of the Government's decision to approve the plans.
They argued that the Government's NPS setting out its support for the project failed to properly deal with the impact on air quality, climate change, noise and congestion.
Support from Labour MPs helped push through the proposals to expand Europe's busiest airport with an overwhelming majority of 296 in a Commons vote in June last year.
Mr Grayling said at the time that the new runway would set a 'clear path to our future as a global nation in the post-Brexit world'.
Heathrow says it could open the third runway 'between early 2028 and late 2029'. Its previous target date was 2026.
The airport said the delay was due to the Civil Aviation Authority, which refused to approve its bid to nearly quadruple the amount it spends before obtaining final planning permission.
The decision prompted joy from environmental campaigners today, but Heathrow has vowed to appeal
Shadow Chancellor John McDonnell called today's decision a 'tremendous victory' for eco campaigners
Mayor of London Sadiq Khan, who supported the legal action, welcomed the decision outside court today
Campaigners gathered outside court before the ruling, from 9am, and reacted joyously after hearing the judges' decision, with one group popping a bottle of champagne.
Mayor of London Sadiq Khan, who supported the legal action, said: 'We won! Today we blocked the Tory government plans to build a third runway at Heathrow Airport. Today's judgment is a major victory for all Londoners who are passionate about tackling the climate emergency and cleaning up our air.
Mr Khan said he was concerned about other issues such as noise pollution, adding: 'I'm worried about future generations.'
'I think what the Government has got to do is realise that the right choice to make is to abandon plans for a third runway,' he said.
Shadow Chancellor John McDonnell joined protesters outside the Court of Appeal and hailed the decision as a 'tremendous victory'.
He added: 'There's no way we can tackle climate change and allow Heathrow expansion to go ahead. I think that project is now dead.'
Environment minister Lord Zac Goldsmith, who lost his seat as MP for Richmond Park in the election last year before being made a peer, simply tweeted: 'HUGE!!!'
The ruling was also greeted by Richmond Council in west London, which has always opposed the plans due to its impact on local residents.
Its leader, Gareth Roberts, said: 'This judgment is nothing short of a victory. It leaves the Government's airports policy in tatters. It surely must be the final nail in the coffin for Heathrow's attempts to steamroll over local and national opposition to their disastrous third runway plans.'
Pictured are plans for what the third runway was due to look like. The future of the project is now unclear
Campaigners also expressed their joy and demanded the third runway be scrapped.
John Sauven, executive director of Greenpeace UK, said: 'The third runway is already on its knees over costs, noise, air pollution, habitat loss and lack of access, and now Heathrow Ltd has yet another impossibly high hurdle to clear.
'No amount of spin from Heathrow's PR machine can obscure the carbon logic of a new runway. Their plans would pollute as much as a small country.
'Boris Johnson should now put Heathrow out of its misery and cancel the third runway once and for all. No ifs, no buts, no lies, no U-turns.'
Aviation Environment Federation deputy director Cait Hewitt said: 'This is a huge win for the climate, and leaves Heathrow's third runway plans in tatters.
'In presenting plans for a third runway to MPs, the Government failed, the court has found, to assess whether this was compatible with the Paris Agreement.'
Tanya Steele, chief executive of the WWF, said: 'No plan for net zero emissions, either from the UK Government or from Heathrow itself, can be credible if it includes a third runway.'
And Will Rundle, head of legal at Friends of the Earth, added: 'This ruling is an absolutely ground-breaking result for climate justice.
'We were fighting a project that would have had dire implications for present and future generations.'
Environmental campaigners hailing their victory outside the Court of Appeal today
However, there was dismay from the aviation lobby, which has supported the project as a means to boost capacity and cement Heathrow's position as a world-leading hub airport.
Tim Alderslade, chief executive of Airlines UK, said: 'Today's decision is extremely disappointing.
'The Sir Howard Davies Airports Commission spent several years looking at airport capacity in the South East and was clear Heathrow is the only game in town, with other schemes being considered and ultimately rejected.
'The economic prize is enormous if expansion is done right, with airlines ready to respond to the unlocking of new capacity by creating new routes and helping to connect the UK to new markets and destinations, and Heathrow to regions across the country.
'UK aviation has committed to net zero carbon by 2050 and this factors in the emissions created by Heathrow expansion. It is not a question of being pro-aviation or pro-environment.'