Theresa May will risk losing Conservative MPs and councils if she backs a new runway at Heathrow as voters will show their anger at the ballot box, a senior Tory has said.
Richmond Park MP Zac Goldsmith said expanding the west London hub would prove to be a "political millstone" for the Prime Minister and urged her to reconsider.
Mrs May is set to make a decision on whether to back a new runway at Heathrow or Gatwick, or both, very shortly, and possibly as early as next week.
Mr Goldsmith, who has promised to resign if Heathrow is expanded, wrote in the Sunday Telegraph: "There is no doubt that despite huge gains against the Liberal Democrats in west London in recent years, the Conservative Party would be badly damaged in the event that Heathrow gets a green light.
"I promised voters I would step down and hold a by-election if Heathrow gets the go-ahead and I will stand by that pledge, but the fallout will be much wider, and it will be hard for our brilliant local councils to weather the storm.
"That's not just a party issue. If I were investing in Heathrow, it would worry me greatly because there is a very real risk that as soon as the polls tighten, the party will feel the need to reverse its policy, yet again."
He added: "But there is cause for optimism, and that is our new Prime Minister. Theresa May is not the kind of person to be spoon-fed lines by entrenched officials.
"If she carefully examines the evidence, she will see that an unbuildable third runway at Heathrow will not only be a political millstone for her premiership and beyond, but, more importantly, will fail the country at a time when business wants to see action."
It comes after Mr Goldsmith said Mrs May would 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 wanted to expand Heathrow, because the pair remain resolutely opposed to the plan.
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.
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, an option Mr Osborne dismissed.
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.