Hundreds of millions of pounds of funding and extra flights to Scotland will be the price for the Scottish National Party supporting a third runway at Heathrow.
Former Scottish first minister Alex Salmond revealed the cost of the support from the SNP’s 55 MPs.
“The question I’ll ask is, ‘What guarantees will you give in terms of connectivity of Scottish destinations into either airport [Heathrow and Gatwick] if they become the choice?’,” he said. “If the answer is, ‘There are no guarantees’, why on earth would we want to support it?
“If people say there are no guarantees they can give, then fine. It’ll be quite easy to make up our minds.”
Prime minister David Cameron was reportedly given the ultimatum as he decides whether to approve expansion of the west London hub.
With a number of Conservative and Labour figures opposed to the move, the government would be expected to seek support from all sides in a Commons vote.
Reports suggested last week that Heathrow was in talks with the SNP to increase air links to Scotland as part of any deal.
In the past decade Heathrow has almost halved the number of weekly services to Scotland, from 50 to 26.
“Heathrow says it’s a private development, but it depends on at least £5 billion of public money, and that’s only the initial estimate,” Salmond told the London Evening Standard.
In a reference to the Barnett formula under which Scotland receives £1 for every £10 of Westminster spending, Salmond added: “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?”
With senior Conservatives, including Boris Johnson, the London mayor, and Zac Goldsmith, the party’s candidate to succeed him, as well as shadow chancellor John McDonnell lined up against Heathrow expansion, SNP opposition to the move would boost the chances of a Commons defeat, the Times reported on Saturday.