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The Court of Appeal is set to make a ruling over Heathrow's expansion in a case described by green groups as massively significant.

Judges will decide whether Heathrow's expansion plans took into account climate change commitments.

If the court rules against the environmentalists, it is likely Heathrow's third runway will be built.

If it rules against the government, ministers could re-start the appraisal process.

This would involve making the highly contentious case that expansion is compatible with combating climate change.

Or the prime minister could also accept a negative verdict and allow the court to take the blame for scuppering the expansion proposal that he has long opposed.

The case has been brought by local residents, councils, the mayor of London, and environmental groups including Greenpeace.

The government’s climate change committee advised that expanding Heathrow is not compatible with a climate neutral economy.

But the former transport secretary Chris Grayling gave the go-ahead to a third runway there in April 2018.

Boris Johnson missed the Commons vote on the scheme. He was in Afghanistan in his role as foreign secretary.

Green groups argue that before the decision was made, Mr Grayling should have taken into account the Paris deal on climate change, which pledged to limit global warming to 1.5 degrees if possible.

At the time, he said: “The step that [the] government is taking today is truly momentous. I am proud that after years of discussion and delay, this government is taking decisive action to secure the UK’s place in the global aviation market – securing jobs and business opportunities for the next decade and beyond.”

Government advisers warned him that expanding aviation would increase emissions when they should be going down.

And since then parliament has agreed to a climate neutral economy by 2050 – substantially more challenging than the 80% emissions reduction target in force when Mr Grayling made his decision.

The green groups don't believe an expanded Heathrow will be able to meet the net zero target, even with the advent of new technologies.

They also think the government’s calculations over Heathrow understate the overall damage aviation does to the climate.

If they win the case, the implications for other government policies in the UK and elsewhere are potentially huge.

Tim Crosland from the pressure group Plan B, one of the organisations which brought the court action, told BBC News: “This would be massively significant – it would mean that in the UK at least carbon-intensive investment shouldn’t happen any more.

“Other nations will be looking at this verdict and taking note [of] what it means to commit to net zero carbon emissions."

'Good for trade'

John Holland-Kaye, chief executive of Heathrow Airport, told BBC Radio 4's Today programme on Wednesday that the airport would play an essential part in post-Brexit Britain.

"Let's be clear, no Heathrow expansion, no global Britain," he said. "That's how simple it is."

He said only a "hub" airport can get goods and people to "all the big trading markets of the world".

"If we're not flying through Heathrow, we'll be flying through Paris Charles De Gaulle," he said. "We'll be handing control of our trading economy to the French - once our friends and partners, now our rivals."

"Now, no prime minister is going to give control of the economy to the French," he said. "We cannot let the French control our trading future."

The government declined to comment.

Follow Roger on Twitter @rharrabin