Balancing act as ministers at odds over airport expansion
Airport expansion in the South East is an issue which divides the Cabinet, with disagreements among senior Conservatives widely blamed for holding back development at Heathrow for years.
Boris Johnson and Justine Greening are fervent opponents of a third runway - with the Foreign Secretary once promising to "lie down in front of the bulldozers" to block construction at the west London airport.
Chancellor Philip Hammond has voiced his preference for expanding Gatwick instead.
If Prime Minister Theresa May gives the green light to Heathrow, as expected, she faces a delicate balancing act to secure parliamentary approval without losing any of her ministers.
Unconfirmed reports suggest that she may offer a free vote to allow ministers to obey the dictates of their consciences - and the wishes of their constituencies - without having to resign.
It has even been rumoured that a means could be found to allow individual ministers such as Mr Johnson or Ms Greening to be absent from Westminster at the time of the crucial vote.
However it is likely that she will lose a high-profile MP, as former London mayoral candidate Zac Goldsmith has repeatedly said he will resign and force a by-election in his Richmond Park seat if Heathrow expansion goes ahead.
Approval for a third runway would mark the completion of a slow and tortuous Tory U-turn since Mrs May's predecessor David Cameron said in 2009: "The third runway at Heathrow is not going ahead, no ifs, no buts."
The Conservative manifesto for the 2010 general election promised to "stop the third runway" - which had been approved by Labour the year before - and instead "make Heathrow airport better, not bigger".
Indeed, Mrs May herself echoed this language on her own website in 2009, when she said that the go-ahead from then transport secretary Geoff Hoon would mean "increased noise and more pollution" for thousands in her Berkshire constituency of Maidenhead, and insisted: "We need a better Heathrow, not a bigger Heathrow".
One of the first decisions of the incoming coalition government in 2010 was to cancel the planned runway, and Mr Cameron then set up an Airports Commission under Sir Howard Davies in a bid to take the politics out of the issue.
Mr Johnson fought hard while Mayor of London for a "Boris Island" alternative of a brand-new purpose-built airport in the Thames Estuary, which was rejected by the Davies Commission as too risky and expensive.
Since becoming MP for Uxbridge and South Ruislip - just a few miles from the airport - he has maintained his opposition, denouncing the third runway as "a ginormous, short-sighted and environmentally catastrophic red herring slap bang in the western suburbs of our city".
Ms Greening, whose Putney seat lies under Heathrow flight-paths, said earlier this year: "Trying to expand Heathrow is like trying to build an eight bedroom mansion on the site of a terraced house. It is a hub airport that is just simply in the wrong place."
While the threat of more flights and more noise at Heathrow has united west London MPs in opposition, there is similar resistance south of the capital to a second runway for Gatwick.
Tory MPs Crispin Blunt (Reigate), Sir Paul Beresford (Mole Valley), Sir Nicholas Soames, (Mid Sussex), Nick Herbert (Arundel and South Downs), Jeremy Quin (Horsham), Henry Smith (Crawley), Nus Ghani (Wealden) and Tom Tugendhat (Tonbridge and Malling) are campaigning against it.
Whatever decision she makes, one thing is certain - Mrs May will not be able to unite her own MPs behind one of the most controversial decisions of her premiership so far.