Boris Johnson has been challenged to resign as Foreign Secretary “on a point of principle” over Heathrow’s third runway by one of his fellow Conservative MPs.

Senior backbencher Sarah Wollaston said that allowing Mr Johnson, a long-term critic of the expansion – to travel abroad to allow him to avoid a three-line whip in favour of the scheme “won’t wash”.

She challenged the former London mayor to “put his money where his mouth is” and stand down, 24 hours ahead of a vote that is likely to expose splits in both Conservative and Labour ranks.

The spotlight will be on the whereabouts of Boris Johnson before and during Monday’s Commons vote on whether to increase capacity at Europe’s busiest airport.

Speaking to the BBC’s Westminster Hour on Sunday, Totnes MP Dr Wollaston pointed out that Greg Hands last week resigned as international trade minister to vote against the Heathrow whip.

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

The resignation of Chelsea and Fulham MP Mr Hands put pressure on Mr Johnson, who once threatened to lie down in front of the bulldozers if the third runway was approved.

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

AIR Heathrow

The Government has so far declined to say where Mr Johnson will be on security grounds.

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

On Sunday night Mr Hands, who went to Romania after stepping down, tweeted: “Great to arrive back in the UK at Luton Airport in time for the match today and to vote against #Heathrow expansion tomorrow. I wouldn’t want to be abroad for either of those. #commitments.”

Following the announcement earlier this month that the Government intended to press ahead with a third runway, Downing Street indicated ministers with long-standing objections would be able to voice their opposition at a “local level” but would not be permitted to speak against it in the Commons.

International Trade Secretary Liam Fox, speaking to Sky News’ Sophy Ridge on Sunday, said: “The Foreign Secretary is regularly out of the country, probably the only person more than myself who is required to be away to do their job. I’ll happen to be there, but I was away last week.”

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

Meanwhile, more than 40 Labour MPs have said they will disregard their frontbench and support a third runway at Heathrow.

The group whose constituencies span the country put their names to a letter to colleagues in the party urging them to support a project they say could create 180,000 jobs across the UK.

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.

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 critics of the party leadership, including Luciana Berger, John Mann, Mike Gapes and Wes Streeting.

They wrote that supporting the scheme is right “in principle”, saying: “It’ll create up to 180,000 new jobs across the country, delivering growth and connectivity for our constituents.

As this project will span multiple parliaments – including, we hope, a Labour government – it’s our responsibility to secure strong cross-party backing for this project.”

Labour has previously said that the expansion plan failed to meet its four tests for support: increased capacity, C02 reduction, minimised noise and shared benefits across the UK.

But the MPs disagreed, writing: “Monday night’s vote is not a blank cheque – the huge benefits from expansion can only be achieved if Heathrow also meets stringent tests on air quality and noise.

“We will work to ensure legally binding safeguards are in place that will mean a new runway can only be built if it is environmentally sustainable.”

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

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.

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.