BORIS Johnson has been mocked and branded “spineless” after he flew 3,500 miles to Afghanistan to avoid the crunch Commons vote on Heathrow.

Shouts of "Where's Boris?" were heard as MPs began the Commons debate on the third runway issue.

Collective responsibility and a three-line whip would almost certainly have meant that if the Foreign Secretary had voted against the UK Government on such a flagship policy, he would have had to resign or be sacked.

READ MORE: SNP accused of putting party interests before national interests by abstaining on Heathrow vote

Mr Johnson has been a long-time opponent of expanding Heathrow; his Uxbridge and South Ruislip would be affected by it. He once famously said he would lie down in front of the bulldozers to stop the third runway from happening.

But in an open letter to constituents in his west London seat, Mr Johnson declared: "My resignation would have achieved absolutely nothing."

Yet his Conservative colleague, Greg Hands, the MP for Chelsea and Fulham, who is also an ardent opponent of a third runway, quit as a minister in the International Trade Department last week. He poured more pressure on the Secretary of State after insisting politicians should honour their promises.

He told MPs: "It's also a debate about being true to your word and to your election pledges."

Earlier, Justine Greening, the former Education Secretary,who is a staunch opponent of Heathrow expansion, took a swipe at Mr Johnson with a tweet welcoming Mr Hands's decision to fly back to the UK to attend the Commons vote.

READ MORE: SNP accused of putting party interests before national interests by abstaining on Heathrow vote

"Great you're back Greg! I wouldn't want any long-term MP campaigners against #Heathrow expansion to miss their chance to represent their community. #commitment #bulldozer," tweeted the Putney MP.

But No 10 had defended the Foreign Secretary, saying that Theresa May believed him to be an “honourable man”.

In Kabul, the Afghan Ministry of Foreign Affairs confirmed Mr Johnson had met deputy foreign minister Hekmat Khalil Karzai for talks; the Foreign Office had earlier refused to disclose his destination on "security grounds".

The Secretary of State’s action opened him up to a savage onslaught from Labour.

Emily Thornberry, his opposite number, said: “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.

READ MORE: SNP accused of putting party interests before national interests by abstaining on Heathrow vote

Jeremy Corbyn, the Labour leader, pressed on whether Mr Johnson should quit over the issue, said: "If he is unable to be present, then we have to ask the question what on Earth is he doing and who is he representing?”

In his letter to constituents, the Foreign Secretary stressed how he had been urged by local councillors to carry on in office so that he could continue to oppose a third runway from within the Government.

He added: "On election night, I promised with John McDonnell, the Labour MP, to lie in front of the bulldozers.

"In view of the very considerable difficulties that still face the third runway, its cost and the appalling air and noise pollution entailed by the project, it will be a very long time before we have to make good on that pledge; if indeed a third runway ever comes about."

On Tuesday, Mr Johnson is due to take Foreign Office Questions in the Commons.