Theresa May has been urged to give the green light to a new runway at Heathrow by 50 home nations' politicians who describe the airport as "our gateway to the world".
The Prime Minister is due to make a decision imminently and is very unlikely to back a compromise option of new runways at both Heathrow and Gatwick, it is understood.
The decision is highly politically sensitive given the opposition to Heathrow expansion within her own Cabinet and party, and it was repeatedly delayed by her predecessor David Cameron.
Now MPs and representatives from the devolved assemblies in Scotland, Wales and Northern Ireland have called on the PM to back expansion of the west London hub.
The group, which includes MPs such as Welsh former Cabinet minister Stephen Crabb, Northern Irish DUP deputy leader Nigel Dodds and Welsh Labour's Stephen Kinnock, wrote in a letter to Mrs May: "Although Heathrow is in London, it is the UK's hub airport.
"Other countries are investing in and supporting their hub, or looking on enviously as they hub through other countries.
"Northern Ireland, Scotland and Wales all have our own successful and growing airports.
"But Heathrow is our gateway to the world, bringing tourists to our attractions and helping our exporters reach new markets.
"Whether we are connected by road, rail or air, we know that connections to Heathrow are a key driver of investment decisions.
"Expanding Heathrow will create nearly four times more jobs in our three nations than other options being considered.
"Heathrow expansion is important for each of our nations remaining open, outward-looking, and trading globally.
"We urge you to act in the interests of all the nations of the UK and urgently approve the expansion of Heathrow."
Mrs May's Cabinet sub-committee on airports is due to meet very shortly, possibly as early as this week, to examine the arguments for the three options set out in last year's Airports Commission report.
The Commission recommended in July 2015 that a third runway should be built at Heathrow.
Other shortlisted options are extending the airport's existing northern runway or building a second runway at Gatwick.
Tory MP Zac Goldsmith has warned Mrs May she faces losing Conservative MPs and councils if she goes for Heathrow.
The unsuccessful London mayoral candidate, who has promised to quit the Commons if Mrs May goes for Heathrow, said expanding the west-London hub would prove to be a "political millstone" for the Prime Minister.
Writing in The Sunday Telegraph, he said: "There is no doubt that despite huge gains against the Liberal Democrats in west London in recent years, the Conservative Party would be badly damaged in the event that Heathrow gets a green light.
"I promised voters I would step down and hold a by-election if Heathrow gets the go-ahead and I will stand by that pledge, but the fallout will be much wider, and it will be hard for our brilliant local councils to weather the storm.
"That's not just a party issue. If I were investing in Heathrow, it would worry me greatly because there is a very real risk that as soon as the polls tighten, the party will feel the need to reverse its policy, yet again."
ends
Members of Unite will deliver a letter to 10 Downing Street on Monday signed by more than 125 reps, representing hundreds of thousands of workers, calling on the Prime Minister to approve Heathrow's expansion.
The letter urges the Prime Minister to be "bold and build", saying: "Giving the green light to Heathrow expansion will not only signal to the rest of the world that the UK is open for business post-Brexit, it is a big test of how serious the Government is about turning its soundbites on industrial strategy and infrastructure, backed by public investment, into reality."
Unite backs Heathrow expansion and is calling on the Government to commit to using domestic manufacturers and steel to maximise the economic dividend and secure decent, well-paid jobs.
Unite London and Eastern regional secretary Peter Kavanagh said: "From Scotland to Plymouth, London to Wales, our members, the country's builders, steelmakers, shipbuilders, aviation and aerospace workers, back Heathrow expansion for the jobs and economic prosperity that it will bring to every corner of the UK.
"Now we need the Prime Minister to be bold and build.
"Coming so soon after the Brexit vote and the dithering over Hinkley, it is a decision that will signal just how serious Theresa May and her Government is about turning her rhetoric on industrial strategy and infrastructure investment into reality.
"In our post-Brexit world, we can ill afford to ignore the economic benefits of expansion. The entire economy will benefit from the spoils of Heathrow expansion - a globally-competitive hub is as essential for our regional and national economies as it is for the capital's.
"Our members' livelihoods and that of their families depend on the continued success of Heathrow. Its expansion will create 180,000 jobs and 10,000 apprenticeships and a £211 billion economic boost across the entire UK economy."
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