SCOTLAND will have to be in line for a £500 million windfall before SNP MPs could support any airport expansion in the south of England, Alex Salmond has warned David Cameron.

The former First Minister explained that creating an extra runway at Heathrow or Gatwick would involve at least £5 billion of public money and so must result in the Holyrood Government receiving its so-called Barnett Formula consequential; estimated to be around £500m.

The MP for Gordon claimed the rival airports were “desperate” to talk to the SNP leadership in their bid to secure the prized airport expansion; executives from each airport have been at the party’s conference in Aberdeen this week, vying for the favour of Nationalist leaders.

Mr Salmond and SNP colleagues have said the backing of the party’s MPs – which could prove crucial in any Commons vote now that there are 55 of them – would rely on getting extra connecting flights from the London airports to Scotland.

However, the ex-party leader made clear that the Prime Minister would have to put cash on the table for Scotland before he could rely on the backing of SNP MPs.

“Heathrow says it’s a private development but it depends on at least £5bn of public money and that’s only the initial estimate,” said Mr Salmond. “What we’d want to know is that if it were to be a development which depended on infrastructure spending, is that spending going to be properly Barnetted or is it going to be another fiddle like the Olympics?”

He noted how only a small amount of money spent on the London 2012 Games resulted in Barnett consequentials, which meant Scotland lost out.

But the ex-FM, pointing to the SNP now being Britain’s third party, said: “Then, of course, that was in a different political situation; there’s a lot more influence now.”

The SNP’s foreign affairs spokesman said he was due to meet with airport representatives in the next day or so and would be asking them what guarantees they could give on connected flights to Scotland.

“We have to start talking specifics, not vague generalities. If people say there are no guarantees they can give, then fine. It’ll be quite easy to make up our minds,” warned Mr Salmond.

A spokesman for Heathrow pointed out that expansion of the airport would be wholly financed through the private sector but that there would be public transport upgrades.

“There is precedent that surface transport investment in London is considered by the Treasury as ‘England only’ and would, therefore, entitle the Scottish Government to reimbursement under Barnett Formula consequentials,” he added.

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, Heathrow rather than Gatwick should be extended.

Mr 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.

The issue of airport expansion is proving highly contentious at Westminster.

A third runway at Heathrow is opposed by many Conservative MPs, including Zac Goldsmith, the Tories’ candidate for London Mayor, Boris Johnson, the incumbent, 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.