Theresa May will have to offer the Tories a free vote or give leave for Boris Johnson and Justine Greening to be absent from the Commons if she backs a new runway at Heathrow, a senior MP in the party has said.
Zac Goldsmith, who failed in his bid to become London Mayor this year, said the Foreign Secretary and Education Secretary would not back expansion of Britain's largest airport under any circumstances.
The pair both represent west London constituencies likely to be affected by Heathrow expansion and remain opposed to the idea, he said.
The Prime Minister is widely tipped to make a decision on whether to back a new runway at Heathrow or Gatwick, or both, next week.
The situation could prove awkward for the pair and Mrs May as Cabinet ministers are supposed to uphold collective responsibility and back the Government's decisions, including in Commons votes.
Mr Goldsmith, who has promised to resign as Richmond Park MP if Heathrow expansion gets the go-ahead, told BBC Two's Newsnight: "I can make a prediction that Boris under no circumstances will ever vote for Heathrow expansion, and nor would Justine Greening - both of them very senior, very valued members of the Cabinet.
"I speak to them both regularly, I spoke to Boris a couple of hours ago, and there is no wavering in his position and there is no wavering in Justine's position.
"They have big jobs to do, they may not be doing the media rounds, they probably won't be talking about Heathrow on Newsnight, but their positions remain absolutely unchanged.
"I absolutely know that.
"They are the major allies."
It comes after former chancellor George Osborne said the west London hub needs increased capacity so that Britain can be "outward-looking, free-trading and global".
Reports emerged on Thursday that ministers could back expansion at both Heathrow and Gatwick.
Mrs May confirmed last week that the Government would "shortly announce" a decision on which expansion project will get the go-ahead.
The Davies Commission recommended in July last year that a third runway should be built at Heathrow.
Other shortlisted options are extending the airport's existing northern runway or building a second runway at Gatwick.
The decision has been repeatedly delayed due to its political sensitivity.
Greenpeace has described expansion of both Heathrow and Gatwick as "the worst of both worlds".
Aviation Minister Lord Ahmad, asked in the House of Lords earlier this week whether the Government will consider supporting one of the two Heathrow options as well as Gatwick, replied: "As far as expansion is concerned, the Commission reported back on the need to increase capacity by 2030 with the addition of one runway in the South East, and that is where the Government's decision is focused."