Zac Goldsmith has reiterated that he will quit his role as a Tory MP if the Government gives the green light to a third runway at Heathrow.
There is mounting speculation the Government is set to back a third runway in the coming weeks, ending years of rowing about airport expansion.
Mr Goldsmith, the MP for Richmond, has been one of the leading figures against Heathrow expansion.
The former London mayoral candidate confirmed that he would stand down if Heathrow expansion was approved - but said the project faced "huge question marks" around its delivery.
Speaking on the Today programme on BBC Radio 4, Mr Goldsmith said: "My position is the same as it was nine years ago.
"I promised my constituents before I was ever elected in 2010 that I would trigger a by-election were the Government to give a green light to Heathrow expansion. My position has never changed.
"I'm committed to my constituents, I'm committed to fighting this campaign and I'm committed to winning this campaign - and I have every confident that will happen."
Mr Goldsmith added: "There are huge question marks about whether a third runway could even be delivered, irrespective of the Government's decision in the next week, two weeks, three weeks or so.
"The legal risks are immense. You've got the councils under the flight path lining up with big cheque books waiting to challenge these proposals.
"There are huge planning risks, there are financial risks.
"Heathrow is massively over leveraged already, there is no prospect of their being able to raise the finance necessary to deliver this project without turning to the taxpayer.
"Whatever the Government decides, this project is not going to happen. In five years' time, 10 years' time, we'll still be arguing about it."
A Heathrow spokeswoman said: "The Independent Airports Commission confirmed that Heathrow expansion could provide the capacity the UK needs more 'easily and quickly' than any other option.
"Our shareholders include three of the world's top five sovereign wealth funds, as well as a UK pension fund, and the world's largest privately funded infrastructure fund - all ready to deliver a shot in the arm of private investment for Britain.
"Generating up to 180,000 jobs and £211bn in national economic benefit, and supported by a majority of our local community, it's time for the Government to finally grasp the nettle and make the right choice to expand Heathrow."