THE SNP could decide whether Heathrow or Gatwick airports should be expanded after the party hierarchy made clear their MPs would vote for whichever option gave the best deal for Scottish travellers.

With the UK Government having a majority of just 12 and Conservative MPs split over whether the plans should go ahead, the 56 Nationalist MPs might hold the key to settling the controversial issue.

But Zac Goldsmith, the Tory MP for Richmond Park and London mayoral frontrunner, 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 it would be right for the Scottish Nationalists to abstain on the forthcoming vote at Westminster on whether to build a third runway at Heathrow or a second at Gatwick.

Earlier this month, the Davies Commission, after nearly three years of deliberation, recommended expanding Heathrow at a cost of £19bn. David Cameron responded by saying the UK Government would examine the lengthy report and give its verdict later in the year. In 2009, the Prime Minister famously declared that there would be no third runway at Heathrow “no ifs, no buts”.

A number of Cabinet Ministers, whose constituencies are in the west of London, have spoken out against a third runway at Heathrow; most notably Theresa May, Philip Hammond, Justine Greening and Theresa Villiers. London Mayor Boris Johnson, whose Westminster constituency is Uxbridge in north west London, is also strongly opposed as is Mr Goldsmith.

That leaves the Nationalists wielding considerable influence.

Drew Hendry, the SNP Transport Spokesman, pointed out his party was currently “absolutely neutral” on whether Heathrow or Gatwick should be expanded but he made clear it would support the option, which answered Scottish demands for cheaper ticket prices and guaranteed connections to international flights.

“The key thing for us is that for too long Scotland has done pretty badly in terms of maintaining routes to regional airports, particularly Inverness and even Aberdeen,” explained the MP for Inverness, Nairn, Badenoch and Strathspey.

Noting how ticket prices were often higher for Scots going abroad via London, he stressed: “The first thing we will be looking for is a fair deal in terms of onward flights. It is costing Scottish consumers much more per head. Onward connections are much more expensive; the prices are ridiculous.”

But Mr Hendry, noting the importance of prices and flight connections, went on: “That does not mean we do not care about environmental issues or the effect it would have on people locally. Whatever decision must be made in the most balanced way but my primary job is to represent the people of Scotland.”

He stressed that the Nationalists did not want locals to suffer undue noise and there was concern about minimising the UK’s greenhouse emissions.

It has been suggested that the SNP’s consideration of both options will dismay bosses at Heathrow, who, it is claimed, had assumed Nicola Sturgeon’s party would back airport expansion to the west of London. Heathrow lobbyists have been campaigning for a free vote at Westminster, believing Scottish MPs and Northern Irish ones would back the Heathrow option as it offered better connections as the UK’s hub airport.

But Mr Hendry made clear he would be negotiating with both airports as well as the Conservative Government to get the best deal for Scottish travellers.