THE issue of whether or not to expand Heathrow airport is a “national decision” and should fully involve the SNP at Westminster, Zac Goldsmith, the Conservative mayoral candidate, has insisted.

In July, Mark Field, the Tory MP for the Cities of London and Westminster, argued that the SNP should abstain on a Westminster vote on locating a new runway in the south east of England.

And Mr Goldsmith was quoted at the time as saying SNP MPs would be “crossing the line in terms of our democracy” if a deal were struck for cheaper flights for Scottish travellers in return for Nationalist votes.

But asked during a lunch for Westminster correspondents if he had concerns that SNP MPs – who, unlike other parties on the divisive issue, were likely to vote as a bloc – might determine whether Heathrow or Gatwick is expanded, the Tory MP for Richmond Park said that airport expansion was one for all MPs to decide, irrespective of the English Votes for English Laws rule.

“It’s not an English decision or a west London decision, it’s a national decision,” declared Mr Goldsmith.

“Heathrow, like Gatwick and Stansted, is a piece of national infrastructure and, therefore, when it comes to making a decision, of course, the SNP should be invited to the table and they should be asked to vote just as I will be, if I’m an MP at the time. So I don’t have any problems with that at all,” he added.

It is now believed David Cameron is set to punt the decision on airport expansion into the long grass with a final decision not due until after the May mayoral election. This would be seen as an attempt to defuse the issue and help Mr Goldsmith’s chances.

The MP, like several of the Prime Minister’s Cabinet colleagues, London mayor Boris Johnson and the Labour leadership, is deeply opposed to creating a third runway at Heathrow. Indeed, Mr Goldsmith has threatened to resign his seat and spark a by-election if the green light is given to expand Heathrow; such a decision, he insisted, would be an “enormous betrayal”.

The PM, who famously said “no ifs, no buts, no third runway” is facing one of the toughest decisions of his premiership given that the Davies report, which the UK Government commissioned, backed the Heathrow option.

Mr Goldsmith told the Westminster lunch that the plan to expand Heathrow was based on "fraudulent" pollution data.

He insisted only a delay based on environmental concerns would be "legitimate" and not appear to be "political game-playing".

Last week, the Commons transport committee said expansion should not be granted until the airport demonstrated it could meet key environmental conditions on climate change, air quality and noise.

Senior ministers will meet tomorrow to plan their next move.