Legal challenges were also brought by London Mayor Sadiq Khan, various borough councils and environmental groups including Greenpeace and Friends of the Earth against the new runway, which MPs had voted in favour of in 2018.
The Court of Appeal has ruled that the government's Airports National Policy Statement (ANPS) - which effectively gave the go-ahead for a third runway at Heathrow - was unlawful as it failed "to take into account the Government's commitment to the provisions of the Paris Agreement on climate change".
After entering the Cabinet as Foreign Secretary in July 2016, Mr Johnson was bound by collective responsibility and he remained in place when the Government approved the Heathrow plan later that year, as his friend and fellow west London MP Zac Goldsmith quit Parliament to force a by-election on the issue - a by-election the now Lord Goldsmith lost heavily.
Heathrow Airport, owned by Spain's Ferrovial, the Qatar Investment Authority and China Investment Corp, has said that it will appeal the decision in the Supreme Court, even though the government is not.
The judgment does state that this ruling does not mean Heathrow expansion should not be delivered, or that the Airports National Policy Statement (ANPS) is not compatible with the UK's commitment to reducing carbon emissions.
On Thursday, Secretary of State for Transport Grant Shapps posted on Twitter, "Airport expansion is core to boosting global connectivity". The government will have to have another go at it, this time taking into account its climate change policies as required by planning legislation.
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THE future of Heathrow expansion is uncertain after leading judges ruled that the government must reconsider its support for a third runway because of the environmental impact.
Heathrow executives said they would challenge the decision. "We face a climate emergency and I'm delighted that the Court of Appeal has recognized that the government can not ignore its climate change responsibilities".
'In our case we think they were wrong on competition grounds, ' Heathrow Hub representative and DAC Beachcroft partner Christopher Stanwell told Legal Business.
The judge added that the government did not oppose its declaration - and had not sought permission to appeal to London's Supreme Court.
Prime Minister Boris Johnson's administration said it wouldn't appeal the ruling, but it could, in theory, return with a new proposal for the expansion that does address climate targets. "However, in the short term, Heathrow is in limbo and as the Prime Minister has announced that he does not plan to appeal against the ruling, this will undoubtedly also call into question aviation expansion in the United Kingdom more generally - there is now no viable technical fix to make planes zero-carbon". Heathrow Airport remains defiant following the judgment, saying that judicial reviews are a common feature of major infrastructure projects, adding that this announcement explicitly does not prevent Heathrow expansion from moving ahead. The UK's Prime Minister, Boris Johnson, is against plans to expand Heathrow, and spent a period of time in his position as London Mayor campaigning for an all new airport to be built in the Thames Estuary instead.
Heathrow says a third runway is needed to meet increasing demand for air travel. "Let's get Heathrow done".