The court of appeal has ruled a government decision to grant permission for a third runway at London Heathrow Airport was illegal.
A case against government approval for the scheme was brought by environmental groups, councils and the mayor of London.
They argued the plan to develop Heathrow was in conflict with environmental commitments made by the UK.
Judges agreed and found the government had not followed UK policy when backing the expansion plans.
Officials had a duty to consider the Paris climate agreement, which seeks to limit global warming.
It was this “legally fatal” to the Heathrow expansion policy that it did not take those climate commitments into account, the judges said.
However, they added a third runway could go ahead in the future, as long as plans fit with climate commitments.
Responding to the decision, a Heathrow spokesperson 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.”