In a victory for climate change campaigners, an appeal court judge said a failure to take into account the government's commitments on climate change was "legally fatal" to the plans to build a third runway at Europe's busiest airport.

Prime Minister Boris Johnson has been a vociferous opponent of the expansion, which was approved under the previous government.

During last year's election campaign, he said he would have to find some way of honouring an old promise to lie down in front of bulldozers at Heathrow to halt the plans.

The government said it would not appeal the ruling, raising questions about the future of a project, which has been hotly debated in Britain for almost half a century.

In order for the new runway to now proceed, the Department for Transport would need to amend its policy statement but it was silent on the matter on Thursday.

"We take seriously our commitments on the environment, clean air and reducing carbon emissions. We will carefully consider this complex judgment and set out our next steps in due course," Transport Minister Grant Shapps said.

Heathrow and its supporters argue Britain's exit from the European Union makes the expansion project critical to ensuring the country can increase trade with the rest of the world.

Heathrow is the United Kingdom's biggest port by value but its two runways are operating at full capacity and the airport will in two years be eclipsed as Europe's busiest by Paris after previous attempts to expand it failed.

Court of Appeal judge Keith Lindblom said that in its current form, the government's policy failed to take into account climate change commitments made by the government when it signed up to the Paris Agreement in 2015.

The government would have to rework the policy if it wants it to go ahead, he said.

Heathrow Airport, owned by Spain's Ferrovial, the Qatar Investment Authority and China Investment Corp, has said it will appeal the decision in the Supreme Court, even though the government is not.

Australian Associated Press