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MPs will decide whether to build a third runway at Heathrow airport later, in a vote likely to expose splits in both Labour and Tory ranks.
The spotlight will be on Boris Johnson, who once vowed to lie down in front of bulldozers to stop Heathrow expansion.
The foreign secretary is facing criticism for missing the vote, as he will be promoting UK trade abroad.
Other Tory MPs will be ordered to back a third runway with Labour MPs given a free vote.
With the government issuing a three-line whip to its MPs - and about 40 Labour MPs expected to vote with them - it is widely expected that the expansion plan will get Commons backing.
But the SNP, which had been expected to vote with the government, could decide to vote against expansion, sources have told the BBC - and the votes of those 35 MPs could make it closer than previously expected.
Chelsea and Fulham Conservative MP Greg Hands quit as international trade minister last week to oppose the airport expansion.
Labour's shadow chancellor, John McDonnell, whose Hayes and Harlington constituency is near the south-west London airport, is expected to vote against expansion, despite Labour's trade union backers being in favour of it.
Opponents of Heathrow expansion have attacked the scheme on environmental, noise and financial grounds, with some making the case for an alternative expansion scheme at Gatwick airport.
Greater Manchester's Labour mayor Andy Burnham urged the prime minister to guarantee funding for transport projects in the north of England - including rail - saying the government's "focus has drifted southwards once again".
However, Transport Secretary Chris Grayling urged MPs of all parties to back "the biggest transport decision in a generation".
He said: "Successive governments have wrestled with the issue of Heathrow expansion, but never before has Parliament held a vote on this project.
"At stake are thousands of new jobs and the country's ability to compete on an international stage and win new global trade."
He made five pledges over the Heathrow expansion:
- No cost to taxpayers
- An economic boost providing 100,000 jobs
- Guaranteed benefits for the whole country including internal flights, rail links and "global opportunities" for regional firms
- Built-in environmental protections
- The ability to fine Heathrow or ground aircraft if the airport breaks its own promises over the scheme.
Boris Johnson, who is MP for Uxbridge and South Ruislip in west London, has faced criticism for his absence from the vote, having a been a vocal opponent of a third runway as London mayor, when he was promoting a scheme for a new airport in the Thames estuary.
Green Party MP Caroline Lucas has called on the foreign secretary to resign, while Labour's John McDonnell said his absence from the vote would damage public trust in politics.
Senior Tory backbencher Sarah Wollaston said allowing Mr Johnson to travel abroad "won't wash" and called for him to resign "on a point of principle".
But the foreign secretary was defended by Prime Minister Theresa May, who said he was busy being "the living embodiment of global Britain" abroad.
The government has so far declined to say where Mr Johnson will be when MPs vote on Monday evening, 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.
The Heathrow expansion is forecast to cost about £14bn, and is likely to lead to hundreds of homes being demolished in the nearby villages of Longford, Harmondsworth and Sipson.
A new runway would increase Heathrow's capacity from 85.5m passengers to 130m.
The Department for Transport has previously said no expansion would mean London's five airports reach full capacity by 2034.
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.