Chris Grayling said he is “cautiously optimistic” that the expansion of Heathrow will be backed by MPs as questions continued about Boris Johnson’s opposition to the plan.

The Foreign Secretary is expected to be out of the country and will miss the Commons vote on a third runway at the London airport, although Transport Secretary Mr Grayling said he had “no idea” where his Cabinet colleague would be.

Mr Johnson once said he would lie down in front of bulldozers to prevent construction of the £14 billion runway.

He was challenged on Sunday by a Tory colleague to “put his money where his mouth is” and resign as Foreign Secretary over his opposition to the scheme.

Heathrow: options for new runway

Senior backbencher Sarah Wollaston said that Theresa May’s decision to allow him to avoid her three-line whip in support of the Heathrow plan by going abroad “won’t wash” and called on him to make a “principled decision” to stand down.

Former whip Stephen Crabb told BBC Radio 4’s Westminster Hour that Mr Johnson would “need to look his constituents in the eye and explain where he was on the night of the Heathrow vote”.

Mr Grayling said: “I have no idea where Boris is, genuinely no idea where Boris is.”

He added that “the Prime Minister has been very clear that there are people in the party who, for various reasons, have long held views about the airport and we are not going to whip those people into voting”.

But Tory MP Greg Hands quit as international trade minister in order to vote against the plan because Conservatives were being whipped to support the expansion.

He said that he “wouldn’t want to be abroad” for the vote, in a pointed reference to Mr Johnson’s absence.

MPs will decide on Monday evening whether to approve the expansion of Europe’s busiest airport, as more than 40 Labour members said they would go against party policy and support the Government’s decision.

Labour is officially opposed to the expansion but Jeremy Corbyn has allowed MPs a free vote on a measure that is supported by trade unions but opposed by the shadow chancellor John McDonnell.

Mr Grayling told BBC Radio 4’s Today programme he was hopeful the plan would be approved by MPs.

“I’m cautiously optimistic, it’s never over until it’s over and the vote actually happens but there is strong support across the political spectrum on this,” he said.

“It’s not usual for me to find myself campaigning on the same side as Len McCluskey of Unite but he is right in arguing that this is a project that can make a real difference to Britain, to jobs, to connections around the world and indeed to the whole UK because this is not simply a project for London and the south-east, the connections that we create through Heathrow benefit every part of the UK.”

He added that it was “very clearly a private sector project” and “the taxpayer is not going to be paying for the expansion of Heathrow Airport”.

Ahead of the vote, officials said the expansion of Heathrow would create 114,000 extra jobs in the area around the airport by 2030, with an extra 16 million 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 said.

AIR Heathrow

Speaking to the Westminster Hour on Sunday, Totnes MP Dr Wollaston pointed out that Mr Hands had resigned and suggested Mr Johnson should follow.

She said: “I think this would be an opportunity for a colleague like Boris Johnson to actually put his money where his mouth is.”

Mrs May last week confirmed he would miss the vote by being “the living embodiment of global Britain” abroad.

The Government has so far declined to say where Mr Johnson will be on security grounds although reports at the weekend suggested he may be somewhere in Africa.

Boris Johnson
Boris Johnson will be abroad ‘as the living embodiment of global Britain’ when the vote takes place (Matt Cardy/PA)

The number of opposition MPs prepared to vote for Heathrow suggests the Commons vote on Monday night should pass with some ease.

Those who have signed the letter include many leadership critics, including Luciana Berger, John Mann, Mike Gapes and Wes Streeting, who argue it is right to back a scheme that will create jobs and growth.

Opponents have attacked the scheme on environmental, noise and financial grounds grounds, with Friends of the Earth saying it was “morally reprehensible” and would see the enlarged Heathrow emitting as much carbon as the whole of Portugal.

A plane flies over Heathrow
The scheme has been opposed over environmental and noise concerns (Steve Parsons/PA)

Paul McGuinness, chairman of the No 3rd Runway Coalition, said: “Heathrow expansion will be bad for London and bad for Britain.”

The Prime Minister’s official spokesman confirmed that Mr Johnson was “travelling” on Monday, but declined to say where he was or when he would be back.

Pressed by reporters about the Foreign Secretary’s apparent failure to live up to his pledges to oppose Heathrow, the spokesman said that the PM regarded Mr Johnson as “an honourable man”.

The spokesman pointed reporters to Mrs May’s comments at a press conference on Thursday last week, when she said Mr Johnson would spend the day acting as “the living embodiment of global Britain” … “out there actually showing the UK’s continued presence around the world”.