Boris Johnson warned he must keep election promise on Heathrow expansion

Exclusive: Shadow chancellor warns of damage to perceptions of politics if foreign secretary dodges key vote on third runway

Boris Johnson has been warned that breaking his promise to oppose expansion at Heathrow will harm public perceptions of politicians

The shadow chancellor, John McDonnell, whose constituency includes the airport, has told The Independent: “If you don’t adhere to your promises people become sceptical not just about you, but about politics overall.”

MPs are due to vote on the controversial plan to build a third runway at Heathrow on Monday.

The government has accepted the unanimous recommendation of the Davies commission to add another runway at Heathrow, rather than extending the northern runway or building a second at Gatwick.

Tory MPs are on a three-line whip to approve the national policy statement, giving a green light to the project.

But the prime minister has revealed that the foreign secretary, a long-standing opponent to expansion at the airport, will be out of the country and therefore not able to vote.

Theresa May told reporters that Mr Johnson would be “the living embodiment of global Britain” in an unspecified location while parliament was voting on the issue.

The Foreign Office was unable to say where the foreign secretary would be or what he would be doing.

When Mr Johnson was first elected MP for the constituency of Uxbridge and South Ruislip in 2015, he pledged to Mr McDonnell: “I will lie down with you in front of those bulldozers and stop the building, stop the construction of that third runway.”

Outside Uxbridge station, one of the foreign secretary’s constituents, Michael Ereira, said: “He’s my MP. All that stuff about lying in front of the bulldozers is all baloney, faff and rubbish.”

Natasha Harrison said she felt that a third runway would increase choice and reduce fares, but said of the foreign secretary: “He needs to vote if he feels that strongly.”

The shadow chancellor told The Independent: “Even if he’s not going to lie down in front of the bulldozers, I expect him to be there and vote against the third runway, because that’s what he promised us.”

In his campaign literature for the 2015 election, Mr Johnson said: “If you elect me as your MP, I will always represent the views of Uxbridge and South Ruislip.”

The apparent ability of the foreign secretary effortlessly and repeatedly to flout the traditions of collective cabinet responsibility highlights the prime minister’s precarious position as she seeks to steer her party towards an acceptable form of Brexit. Mr Johnson led the Leave campaign and has criticised a number of Ms May’s strategies.

One of her well-regarded junior ministers, Greg Hands, resigned as trade minister in order to vote against the airport expansion plans in the Commons.

Labour says the third runway does not meet the four environmental tests it has set. Yet in a move that has been widely criticised, the party is giving MPs a free vote on the expansion plans.

Mr McDonnell said: “I think, and I’ve said this to all my colleagues, this runway will not be built.

“Even if the legal challenges go down, this will be the totemic or iconic battleground of climate change and you’ll see massive opposition.

“My view, and my advice to Boris is: you promised to oppose the third runway. Come to parliament on Monday and walk through the lobbies with me.”

A Heathrow spokesperson said: “Political support for expanding Heathrow has been growing for years, with the latest independent polling from ComRes showing that 75 per cent of MPs now back a third runway.

“They are joined by Britain’s major business groups, trade unions and more local people who support than oppose who are keen to secure the billions in growth and tens of thousands of new skilled jobs that expanding the UK’s only hub airport will deliver for future generations.”

The airport believes the new runway can be completed by 2026.