Zac Goldsmith has quit as a Conservative MP and will stand as an independent in the by-election he has triggered in protest over Heathrow expansion.
The failed London mayoral candidate said the decision to push ahead with a third runway was "catastrophic".
Mr Goldsmith, a high-profile environmentalist whose Richmond Park and North Kingston constituency lies under the Heathrow flight path, has long threatened to resign if the Government backed the expansion plan.
Constituents feel "let down" down that the promises made by the Tories under David Cameron had not been kept, he said.
Mr Goldsmith insisted the fight to stop expansion was not over and insisted the by-election would be a "chance to send a message to government".
He said: "I promised you if my party won the election, the third runway would be scrapped. And I wasn't making it up.
"My party leader - the then leader of the Opposition - made that same promise directly to us. He came here and told us: 'No ifs, no buts, there will be no third runway'.
"It was music to our ears. But not everyone believed him, because people assume the worst in politicians. So I followed his promise with my own. I told you that if my party changed its position, I would trigger a by-election and give you a chance to vote again.
"There was no small print. No expiry date. No ambiguity. It was a simple promise. And it mattered. I know it mattered, because the thought of Heathrow expansion fills most of my constituents with dread.
"That's why my party's promise mattered. It's why my promise mattered. And it's why so many people in our community feel so let down today."
Mr Goldsmith said the Government had chosen the "most polluting, most disruptive, most expensive option" but it was "also chosen the option with the least chance of being delivered".
"The sheer complexity, cost and legal difficulties mean it is unlikely ever to happen," he said. "It will be a millstone round this Government's neck for years."