Media playback is unsupported on your device Media caption "This election must be a referendum on Heathrow expansion" - Zac Goldsmith

Conservative MP Zac Goldsmith has resigned his seat in a protest against the government's backing for a new runway at Heathrow Airport.

His decision forces a by-election in his constituency of Richmond Park, where he will stand as an independent.

The Conservatives said they "disagreed" with Mr Goldsmith's decision but would not field a candidate against him.

The MP and former London mayoral candidate has long campaigned against expanding Heathrow.

The Transport Secretary Chris Grayling has ruled out any rethink of the policy to expand the airport.

The Department for Transport says a new runway will benefit passengers and boost the wider economy by up to £61bn, creating as many as 77,000 local jobs over the next 14 years.

The Richmond Park by-election is expected to take place in December.

Liberal Democrat leader Tim Farron said his party would be "taking the fight" to Mr Goldsmith and "put huge pressure" on the prime minister to abandon her Heathrow plans.

Media playback is unsupported on your device Media caption Transport Secretary Chris Grayling said the government was determined to see the new runway built.

Speaking at a school in his constituency, Mr Goldsmith said the contest 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."

He added: "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."

Mr Goldsmith 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".

He added: "The sheer complexity, cost and legal difficulties mean it is unlikely ever to happen. It will be a millstone round this government's neck for years."

'Manifesto commitment'

But a Conservative spokesman said the government was "taking decisive action in the national interest to secure the UK's place in the global aviation market - securing jobs and business opportunities for the next decade and beyond".

He added: "That's why we have delivered on our manifesto commitment to respond to the Airports Commission's final report and have announced our support for the first full-length runway in the South East since the Second World War.

"We disagree with Zac about the need for a by-election in light of this decision, but understand his position. He has been a hard-working champion for the people of Richmond Park - as we know, he will continue to be if they re-elect him as their Member of Parliament - so we will not be putting up a candidate against him at this by-election."

Earlier Mr Grayling told the House of Commons: "The prime minister has been very clear that she does not want to force - and indeed I don't think the public would expect us to force - MPs with long-standing principles of disagreement over this to go against their own views."

Mr Goldsmith held his south-west London seat in 2015 with a 23,015 majority.

The Liberal Democrats, who held the seat until 2010, finished second.

Mr Farron said the party was looking to "build on our result" in the recent Witney by-election, where it reduced the Conservative majority in David Cameron's former seat to 5,702 - down by more than 20,000 votes.

He added: "A victory for the Liberal Democrats would put huge pressure on Theresa May to abandon her plans for Heathrow expansion and an economically disastrous hard Brexit."

His fellow Lib Dem, former Business Secretary Sir Vince Cable, said there was a danger, if Mr Goldsmith stood "with the support of the Conservatives but nominally independent", of the by-election becoming a "Zac Goldsmith ego-trip".