Concerns over climate change might restrict the growth of flying in the UK, the government has admitted.
The advisory Committee on Climate Change (CCC) recently said the UK's planned increase in aviation would need to be curbed to restrict CO2.
Now a senior civil servant has told a green group that means ministers may have to review aviation strategy.
The group says climate concern is so high the decision on Heathrow expansion should be brought back to Parliament.
The Department for Transport defended the proposed Heathrow expansion, saying it would "provide a massive economic boost to businesses and communities" across the UK, all at "no cost to the taxpayer and within our environmental obligations".
Why should the policy change?
It is a crucial time for flying, with policy on aviation right up to 2050 currently out for consultation.
When the government first laid out proposals for increasing aviation, the UK had an overall target of cutting CO2 emissions by 80% by 2050.
But the CCC recently raised the bar of ambition in recommending that Britain should adopt a target of net zero emissions.
That will mean compensating for any greenhouse gases by either capturing the CO2 and storing it, or planting more trees.
Under the previous 80% scenario, aviation had a privileged position. Its expansion would be counter-balanced by additional CO2 cuts in other sectors, like industry.
The CCC makes it clear this is not an option in a zero-carbon Britain. It says people will continue flying using fuels made from waste, or - in the long-term - electricity.
But crucially, the growth in aviation must be constrained.
The CCC will make further recommendations on this issue in the coming months.
How was the news revealed?
In a letter to a tiny pressure group Plan B, the Department for Transport (DfT) aviation head Caroline Low said: “It may be necessary to consider the CCC’s recommended policy approach for aviation.”
This may sound like a cautious civil servant covering bases, but for Plan B it is an admission that the DfT will have to confront the notion that concerns over climate change may outweigh people’s desire to fly more.
Tim Crosland from Plan B told BBC News: “We're pleased to see the government is taking seriously our request to review the expansion of Heathrow.
“Since the (Heathrow) proposal was approved there have been developments of immense significance.
"The Intergovernmental Panel on Climate Change report spelling out the dire consequences of exceeding 1.5C average global warming; Parliament's recognition of a state of climate and ecological emergency; and the CCC's advice that it is 'necessary' for the UK to reach net zero emissions by 2050," Mr Crosland said.
“The government can either take the necessary action to avoid climate breakdown or it can stick to 'business as usual' and expand aviation, the most polluting mode of transport.
“But it can't have it both ways."
Mr Crosland noted that the Scottish government said this week it would review its support for Heathrow in the light of the CCC’s net zero report.
And he called for Heathrow expansion to be brought back to Parliament.
In response, the DfT said: "We take our commitment to the environment very seriously and we will give careful consideration to the net zero report.
"No decision has been taken to review the Airports National Policy Statement, however we are legally obliged to consider requests like this one."
Prof Kevin Anderson, from Manchester University, told BBC News that curbing the growth in aviation would be politically possible because, in his opinion, most flights are taken by the rich.
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