Zac Goldsmith has quit as Tory MP over the Government's decision to give Heathrow Airport a new runway.
Environmentalist Goldsmith called the move "catastrophic" - and announced he will now stand as an independent in protest.
This will trigger a by-election in his Richmond Park constituency.
But the Conservative Party said they would not put forward another candidate.
Speaking at a press conference, Goldsmith insisted the fight to stop expansion was not over and said the by-election would be a "chance to send a message to government".
He said the Government had chosen the "most polluting, most disruptive, most expensive option" but it had "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."
The Government backed a third runway after rejecting proposals to expand an existing runway at Heathrow or build a second runway at Gatwick.
A public consultation will now be held on the impact of expansion at the west London hub before the final decision is put to MPs for a vote in the winter of 2017/18. A new runway could be operational by 2025.
He said there were "colossal" problems with the proposal and anticipated it would be "snarled up" in legal cases.
"I think it very likely it will be stopped," he added.