BORIS Johnson has defended his decision not to resign over his opposition to the UK Government's plan for a third runway at Heathrow as the SNP Government was called on to make a Holyrood statement on whether it still supported the controversial £14 billion proposal.

The Afghan Foreign Ministry has said the Foreign Secretary is in Afghanistan for talks with local officials as MPs prepare for a crunch Commons vote on its flagship policy, reiterating only that he will be abroad.

Last week, Theresa May confirmed he would miss the vote by being "the living embodiment of global Britain" abroad but on Tuesday morning Mr Johnson is due to lead for the UK Government at Foreign Questions in the Commons.

The former London Mayor, who famously said he would lie down before bulldozers to stop Heathrow’s expansion, today declared: “My resignation would have achieved absolutely nothing.”

The Cabinet Minister, whose Uxbridge and South Ruislip constituency is under the airport’s flight path, made clear he would continue to oppose a third runway, forecasting that it would not be built.

But Mr Johnson has come in for criticism for absenting himself from this evening’s key vote and has been accused of undermining trust in politicians.

Theresa May’s spokesman insisted the Prime Minister continued to believe he was an “honourable man,” pointing out how, because of their long-held opposition to Heathrow expansion, that some ministers had been given leeway to oppose it locally.

Yet the Heathrow vote is under a Government three-line whip, which normally means a minister who does not abide by it resigns or is sacked.

Last week, Mr Johnson’s Conservative colleague Greg Hands, who represents Chelsea and Fulham, and who has also been a vociferous opponent of the third runway plan, resigned his role as Liam Fox’s deputy at the Department for International Trade.

In today’s debate, Mr Hands will pour more pressure on the Foreign Secretary by stressing how it is important politicians keep their promises.

He is due to say: “My pledge could not have been clearer. So my resignation isn’t just about Heathrow, important though that is, it is also about trust in politics and politicians, which is a wider and more pressing imperative.”

Labour launched its own broadside with Emily Thornberry saying: “Boris Johnson likes to call other politicians invertebrate jellies, yet his decision to flee the country rather than make good his previous promises on Heathrow shows him to be the most spineless of all.

“On this issue, he has proven himself utterly devoid of courage, strength, or principle. Never again should he dare to call Winston Churchill his role model,” added the Shadow Foreign Secretary.

Labour’s position on Heathrow expansion is to oppose it because it has not met their four tests on issues such as protecting the environment and regional airports.

But the Opposition is also allowing its MPs to have a free vote, which means dozens are likely to vote with the Government; the Commons arithmetic, therefore, will be in Mrs May’s favour.

Chris Grayling, the Transport Secretary, said he was "cautiously optimistic" that the expansion of Heathrow would win the day.

“It's never over until it's over and the vote actually happens but there is strong support across the political spectrum on this,” noted Mr Grayling.

It also emerged that Chancellor Philip Hammond is abroad on official business in India and will miss the key vote.

Meanwhile, the SNP, which has hitherto been supportive, was still playing its cards close to its chest this morning over which why it would vote in a few hours’ time.

Last week, The Herald reported party sources suggesting it was “possible” the 35 Nationalist MPs might oppose the UK Government’s plan even though Edinburgh had previously come out for it.

In 2016, the SNP administration signed a memorandum of understanding with Heathrow, in which the airport pledged, among other things, to create up to 16,000 new jobs across Scotland because of the increased capacity a third runway would provide and discuss making Glasgow Prestwick Airport a logistics hub to support the expansion of Heathrow.

Today, the Scottish Conservatives demanded an emergency ministerial statement at Holyrood,

Murdo Fraser, the party’s Shadow Finance Secretary, said: "The SNP is in chaos on Heathrow. Keith Brown made clear it was the best option for jobs and the economy just weeks ago. Now it appears SNP MPs are in open revolt.

“The suspicion is that the Nationalists are putting political games before the good of the country. Now we are learning what their threat to disrupt Government business means: thousands of Scottish jobs down the pan.”

Mr Fraser added: "Enough of the hiding. Keith Brown must come before Parliament tomorrow and explain the SNP's behaviour.

“Scottish jobs are at stake and the Nationalists should, for once, put grandstanding and politicking to one side."

At Holyrood, Patrick Harvie for the Greens challenged SNP MPs to resist voting with the Tories on Heathrow expansion, saying it was “incompatible” with environmental and climate change obligations, and urged them instead to focus on improving “public transport at home”.

Ahead of today’s Commons vote, Whitehall officials have said the expansion of Heathrow would create 114,000 extra jobs in the area around the airport by 2030, with an extra 16m long-haul seats by 2040.

It would represent the first full-length runway in the south east since the Second World War, the Department for Transport has said.