A U-turn by David Cameron over Heathrow expansion would represent an "off-the-scale betrayal" which would cause voters across the country to question whether he could be trusted, a Tory MP has said.
The Government-appointed Airports Commission is due to outline its initial thoughts in a report next week, but Tory Zac Goldsmith questioned its independence and suggested that accepting its findings would effectively be supporting the expansion of Heathrow.
Mr Goldsmith, whose Richmond Park & North Kingston seat in west London would be affected by the growth of Heathrow, warned the Prime Minister that abandoning opposition to a third runway at the airport would be "catastrophic" for his reputation.
The MP, a leading opponent of Heathrow expansion, told BBC2's Newsnight that the commission led by Sir Howard Davies was not the "arms-length" review promised by ministers.
He claimed that Chancellor George Osborne had been instrumental in making sure that other options apart from Heathrow would be contained in the report to be published on Tuesday, but only to "cynically" provide political cover to avoid making a final decision until after the next election in 2015.
"This review was always supposed to be an independent, arms-length review. It seems very clear now that it's nothing of the sort," Mr Goldsmith said.
"It looks very much like George Osborne in particular has been knocking it about in the last few days so that what finally emerges on Tuesday will not just be about Heathrow expansion.
"We will have a few other synthetic options thrown in as well just to enable the Government to maintain that ambiguity, cynically I believe, until after the next election."
He added: "This is not genuinely to expand the choices, this is about enabling, I think, all three party leaders to defer any kind of decision-making until after the election because none of them, frankly, have the courage to front up to voters before the election when it really matters."
Mr Goldsmith urged party leaders to "come clean" about what the report meant.
He said: "I think, because of what we know was in the original first draft of this report, irrespective of what is produced on Tuesday, we know that if the parties accept this report in general they are accepting Heathrow expansion.
"I think they need to come clean about that."
The Tory MP has promised to trigger a by-election if his party's position changed and would not stand as a Conservative if the 2015 manifesto supported Heathrow expansion.
" David Cameron himself has to really think very carefully about this. Politically a U-turn on this issue would be catastrophic for him. You have to remember it wasn't just a few party speeches, David Cameron went to every single constituency affected and stood up and said 'no ifs, no buts, there will be no Heathrow expansion'," he said.
"If he does a U-turn on this issue it would be an off-the-scale betrayal and he will never be forgiven in west London.
"People outside of west London who perhaps don't care much about Heathrow will also take note and they will wonder how many of the other promises we will hear between now and the next election can be trusted, how much can the Prime Minister himself be trusted if he is willing to break a promise that was so crystal clear?"