EXECUTIVES from Heathrow are in private talks with the SNP leadership in their bid to win support for the controversial plan to create a £19 billion third runway at London airport.

Securing the backing of the Nationalists’ 55 MPs could prove crucial if there is a House of Commons vote to decide the issue.

But such a scenario would be constitutionally incendiary at Westminster if such a decision on an airport in southern England were effectively determined by MPs from Scotland given the widespread opposition to a third runway at Heathrow from local politicians among the Conservatives, the Labour Party, the Liberal Democrats and the Greens.

Over summer, Zac Goldsmith, the MP for Richmond Park who is the Tories’ London mayoral candidate, warned that SNP MPs would be "crossing the line in terms of democracy" if a deal were struck for cheaper flights for Scottish travellers in return for Nationalist votes at Westminster.

His Conservative colleague Mark Field, who represents the Cities of London and Westminster, said Scottish Nationalists should abstain on any forthcoming vote at Westminster on whether to build a third runway at Heathrow or, indeed, a second one at Gatwick.

In July, the Davies Inquiry, commissioned three years ago by the UK Government, recommended that, if stringent noise and air pollution conditions could be met, then Heathrow rather than Gatwick should be extended.

David Cameron, who in 2010 famously declared that there would be no third runway at Heathrow “no ifs, no buts”, is due to give his response by the end of the year.

But expansion of Heathrow is proving highly contentious.

It is opposed by many Conservative MPs, including Mr Goldsmith, Boris Johnson, the London Mayor, and Cabinet Ministers Philip Hammond, Theresa Villiers and Justice Greening. Labour leader Jeremy Corbyn and his Shadow Chancellor John McDonnell are also strongly opposed; both are London MPs.

While it is estimated that a third runway at Heathrow would mean 250,000 more flights a year, provide a £150bn boost to the UK’s income over the next 60 years and create 70,000 new jobs, it would also mean demolishing almost 800 homes, including most of the nearby village of Harmondsworth.

Drew Hendry, the SNP transport spokesman, has stressed that his party is currently "absolutely neutral" on whether Heathrow or Gatwick should be expanded. But he has made clear it would support the option that answered Scottish demands for cheaper ticket prices and guaranteed connections to international flights.

Lobbyists from Heathrow and Gatwick are at the SNP conference in Aberdeen this week with the former paying for large banners at the conference venue.

Heathrow told the Financial Times that it was in discussions with politicians from across the UK on how to maximise the benefits for their constituents.

“Scotland,” it said, “like all regions and nations of the UK stands to benefit from the 180,000 new skilled jobs and £211bn in economic growth that Heathrow expansion will deliver.”

A senior figure at Heathrow told the FT that there was a clear deal to be cut with the SNP in return for the party’s support.

He said Scottish passengers wanted a firm long-term commitment to protect existing routes such as Aberdeen but also wanted to see the opening of new ones like a route between Heathrow and Dundee or the reopening of the old direct route to Inverness.

Speaking at a Heathrow-sponsored conference fringe event, Stewart Hosie, deputy leader of the SNP and MP for Dundee East, said: “The first airport that offers us connectivity in abundance will be in a prime position to win our support.”