Video report by ITV News Political Correspondent Libby Wiener
The future of Heathrow Airport's third runway is in doubt after judges ruled in favour of campaigners fighting to block the expansion.
The Court of Appeal said on Thursday the Government did not take "enough account" of its commitment to the Paris Agreement on climate change when setting out plans for the expansion.
A spokeswoman for Heathrow said it would appeal the ruling, adding she was "confident that we will be successful."
"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," she said.
Transport Secretary Grant Shapps confirmed the Government would not appeal the decision but insisted Mr Johnson was committed to airport expansion in the UK, if it meets the pledge to be carbon neutral by 2050.
Mr Shapps said: "What happens next will be up to Heathrow. We've always said this will be sector-led, industry led. That's what our manifesto made clear.
"Where and how that expansions is a matter for the industry, but we're clear we want to see airport expansion, but critically we must meet those net zero targets for the next generation."
Campaigners have been fighting a decades-long battle to block construction of the third runway, arguing the Government failed to properly deal with the project's impact on air quality, climate change, noise and congestion.
The Court of Appeal ruled in favour of the Friends of the Earth and Plan B Earth, saying the government hadn't considered its commitments to the Paris Climate Agreement.
Delivering the ruling at the Court of Appeal, Lord Justice Lindblom said: "The Paris Agreement ought to have been taken into account by the Secretary of State in the preparation of the National Policy Statement and an explanation given as to how it was taken into account, but it was not."
He added: "Our decision should be properly understood.
"We have not decided, and could not decide, that there will be no third runway at Heathrow."
Speaking after the ruling, Mr Khan said: "We won... Today's judgement is a major victory for all Londoners who are passionate about tackling the climate emergency and cleaning up our air," he said.
Westminster approved the airport's plans in 2018, but since then Parliament has agreed to a climate neutral economy by 2050 – substantially more challenging than the 80% emissions reduction target that was previously set.
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.
John Sauven, executive director of Greenpeace UK, called on the government to "permanently ground" the expansion following the ruling.
"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," he said.
Why does Heathrow want to expand?
A Government white paper first recommended expanding Heathrow Airport in 2003, and after more than a decade of back-and-forth, MPs voted in favour of expansion in June 2018.
The airport argues both of its current runways are full and have been operating at 98% capacity for more than a decade.
A third runway - at a cost of £14 billion - would enable the airport to handle an additional 260,000 flights per year, keeping it competitive with rival airports in Paris, Frankfurt and Amsterdam.
The case against expansion
Environmental groups have long opposed Heathrow expansion on the grounds that more flights will mean more carbon emissions.
Heathrow argues it has an "ambitious plan" to treble its rail capacity by 2040, encouraging passengers to travel by rail rather than car to the airport.
Local campaign groups expressed grave concerns about noise from additional flights, despite an offer from Heathrow to limit scheduled night flights and install a "world-class" noise insulation scheme for homes close to the airport.
Some airlines have also warned the £14 billion expansion fee could be passed down to customers through an increase of fares, but Heathrow has pledged to keep charges "close to current levels."
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 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."
Prime Minister Boris 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 missed the vote in the Commons on Heathrow's expansion as he was in Afghanistan as Foreign Secretary.
Earlier this month, Mr Johnson said there was "no immediate prospect" of construction beginning.
A High Court ruling in May last year rejected four separate judicial reviews of the Government's decision to approve the plans.
Campaigners appealed against that decision, arguing the government failed to properly deal with their concerns in setting out support for the project.
Before the ruling, Heathrow said it could open the third runway "between early 2028 and late 2029".
Its previous target date was 2026.
Heathrow 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.