BORIS Johnson will, as widely expected, be “out of the country” when MPs on Monday vote on the UK Government’s highly controversial plan to build a third runway at Heathrow.

Due to “security reasons,” the Foreign Office declined to say where the Foreign Secretary would be precisely but Theresa May, confirming her colleague’s absence from the Commons, insisted he was the “living embodiment of global Britain”.

Mr Johnson, a former London mayor, who represents Uxbridge in the north west part of the UK capital, has vociferously opposed the third runway proposal, once declaring how he would “lie down in front of bulldozers” to stop it going ahead.

Collective responsibility means that if the Secretary of State would have voted against the Government plan, then he would have had to resign.

This was the circumstance his colleague Greg Hands, the International Trade Minister, who represents another London seat, Chelsea and Fulham, found himself in.

On Thursday, he resigned because, as he told the Prime Minister in a resignation letter, he intended to vote against the third runway plan next week.

His decision immediately turned the spotlight onto Mr Johnson.

But Mrs May told reporters: “The Foreign Secretary early next week will be, what I would describe as, the living embodiment of global Britain.

"He will be out there actually showing the UK's continued presence around the world and the work that the UK continues to do around the world with our diplomacy, working on so many of the issues and challenges that we face across the world today."

The Commons vote coincides with a meeting of EU foreign ministers in Luxembourg but Sir Alan Duncan, the Europe Minister, is set to be the UK's representative there.

Sir Vince Cable, the Liberal Democrat leader, said: "I hope other ministers, such as Boris Johnson, who have previously taken a staunch anti-Heathrow expansion stance, will now follow the Liberal Democrats and vote against the third runway."

Labour has formally come out against the Heathrow proposal, saying it does not meet its four tests, including helping regional airports, reducing noise and the environmental impact as well as meeting carbon dioxide reduction targets.

But its decision to give its MPs a free vote means the Commons arithmetic will swing in the Government’s favour.

If Labour had whipped its MPs to oppose the third runway, then the SNP could have become the determining factor.

Thus far, it has spoken positively about the third runway; the Scottish Government has a memorandum of understanding with Heathrow.

While one Nationalist MP suggested it was “possible” the SNP group could vote against the Conservative proposal, it is thought senior party figures will use this weekend to finalise their plan of action for Monday, which they have always said would be undertaken “in the interests of Scotland”.