Zac Goldsmith has quit as a Conservative MP and will stand as an independent in the by-election he has triggered in Richmond Park and North Kingston 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, has long threatened to resign if the government backed the expansion plan.

Constituents feel "let down" that the promises made by the Tories under David Cameron had not been kept, he said.

MORE: Detail and reaction after Heathrow Airport expansion gets go-ahead with government choosing third runway

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 has "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 Liberal Democrats are likely to pour resources into the constituency after heavily cutting the Tory majority in the Witney by-election last week in a contest triggered by former prime minister David Cameron standing down as an MP.

Liberal Democrat former business secretary Sir Vince Cable said he was "very, very unlikely" to stand against Mr Goldsmith in Richmond Park but would not rule it out.

"It's unlikely and we have a candidate already in Richmond Park as it happens," the former Twickenham MP told Sky News.

"The key challenge actually is whether the government itself puts up a candidate and if Zac Goldsmith's going to stand as an anti-Heathrow candidate, is Theresa May going to put up her own Conservative, that's the key question we want an answer to."

Asked to rule it out, he said: "I think it's very, very unlikely and we've got a good candidate.

"I'm actually being asked if I'd stand in Twickenham, which is my first loyalty."