Heathrow expansion looks doomed after the government announced it would not challenge a devastating court ruling against the controversial project.

The judgment – that the third runway is “unlawful” by failing to take into account the UK’s climate change commitments – now looks likely to give Boris Johnson a convenient exit route from the scheme.

Asked whether it was the government’s intention to appeal against the decision​, the prime minister’s spokesman replied: “It is not.”

Grant Shapps, the transport secretary, failed to set out a plan B, insisting ministers still wanted “overall airport expansion” – while arguing it was also “absolutely determined” to hit carbon emission commitments.

However, the saga is not over, because Heathrow airport announced it would mount an appeal – despite not being a party to the Court of Appeal case.

“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,” a spokesperson said.

No 10’s decision confirms suspicions that Mr Johnson – who once vowed to “lie down” in front of bulldozers to stop a third runway – could use a defeat to abandon expansion altogether.

In recent weeks, he had suggested the project would be stopped by the courts, telling MPs this month: “I see no bulldozers at present and no immediate prospect of them arriving.”

And he is known to rage at criticism that he fails to stick to his promises – and be conscious that diggers moving into Heathrow would be the ultimate symbol of a broken pledge.

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Friends of the Earth hailed the ruling as “an absolutely groundbreaking result for climate justice”, adding: “We were fighting a project that would have had dire implications for present and future generations.

And Greenpeace predicted defeat for Heathrow’s appeal, saying: “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.”

The landmark decision is first time any government has been held to account over the Paris Accord, said Margaretha Wewerinke-Singh, assistant professor of public international law at Leiden University, in the Netherlands.

The court said that in future, a third runway at Heathrow could go ahead – but only if it fitted with the UK’s commitment under the Paris Agreement, which seeks to limit global heating.

The judges found that the government had not followed that policy when backing the expansion plans two years ago.

It was “legally fatal” to Heathrow expansion that it did not take those climate commitments into account, they said.

That raises huge questions over trying to expand another airport, such as Gatwick or Birmingham – and has implications for all huge infrastructure projects.

Mr Shapps tweeted: “Airport expansion is core to boosting global connectivity. We also take seriously our commitment to the environment.