The Liberal Democrats could be poised to help "change the direction" of the country if they successfully pull off a spectacular by-election victory in Zac Goldsmith's constituency, Tim Farron has said.

Mr Goldsmith has forced the contest in his seat of Richmond Park with his announcement that he is quitting the Conservatives to stand as an independent in protest at the decision to give the green light to a new runway at Heathrow Airport.

Speaking on Richmond Green, directly below the Heathrow flight path, the Lib Dem leader insisted the by-election was also an opportunity to "send a message" to Theresa May over her "hard Brexit" strategy, in a seat that recorded one of the biggest Remain votes in June's referendum.

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He pitched his party as the "only clear, electable alternative" to Mr Goldsmith, repeating his belief that the Labour Party were no longer an adequate opposition.

On the face of it the Lib Dems have a mountain to climb. Mr Goldsmith held the south-west London constituency with an increased majority of more than 23,000 in last year's general election and his stand on Heathrow is likely to be highly popular locally.

But, heartened by a strong performance in David Cameron's former constituency of Witney last week, Mr Farron said his party were determined to take the fight to Mr Goldsmith.

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He told the Press Assocation: "We start here as second favourites, there's no two ways about it, there's a big mountain to climb, but it's one we're determined to have a good crack at.

"The ability for people of Richmond Park to really change the way our country heads is over Brexit - are we going to leave the EU or are we going to leave the single market, risking jobs and livelihoods in this part of London?

"If you want to stop Theresa May taking us down this extreme path that no-one voted for, then a vote for the Liberal Democrats will send a powerful message and could literally change the direction of our country."

Mr Farron suggested he would not be moving to parachute a big name in to contest the seat, saying it was a matter for the local party.

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He also pointed to the Lib Dem's record in Government at stopping the expansion of Heathrow, claiming their departure had been the catalyst for the third runway announcement.

"The reason why the announcement was made this week is because we now have a Conservative Government without the Liberal Democrats stopping them, so the Liberal Democrats are the only people who have stopped the third runway so far," he said.

"It's a confusing choice in some sense, because (Mr Goldsmith), who says he's opposed to Heathrow, is being supported by a party which is making the Heathrow expansion happen.

"To vote for Zac Goldsmith is basically to give succour to a Tory government that is expanding Heathrow."

Activists are set to pour the Richmond Park constituency in a bid to capitalise on their success in last week's by-election.

The Conservative majority in former prime minister Mr Cameron's old seat was slashed from 25,000 to 5,700 with a 19% swing, while the Lib Dems came second despite having finished a distant fourth in 2015.

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They believe Richmond Park - which they held until 2010 - could prove even more fertile ground for their message of the dangers to the economy of a hard Brexit without access to the EU single market.

The constituency voted 69% for Remain in the referendum while Mr Goldsmith was among the leading Conservatives campaigning for Leave.

The local Lib Dem party is one of the biggest in the country with 800 members, and it enjoys support in neighbouring seats as well.

"We are going to put a huge amount into this one," a Lib Dem source said. "The members are really fired up since Witney. We are fully anticipating a vast number of activists sweeping into the constituency."

Earlier, Mr Farron said disclosures about Mrs May's private warnings to bankers before the referendum of the harm leaving the EU would do to the economy underlined how "monumentally damaging" the Government's strategy is.

"This will be a referendum on Theresa May's leadership of the Conservative Brexit Government," he told the Press Association.

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While the Conservatives have decided not to stand against Mr Goldsmith to avoid exacerbating divisions in the party on Heathrow, the Lib Dems believe the move could play into their hands, making it harder for him to portray himself as a genuine independent.

Mr Farron's criticism of Labour comes as the party said it would be fielding a candidate despite calls from its own MPs not do so.

A host of prominent party figures, including shadow business secretary Clive Lewis and shadow City minister Jonathan Reynolds condemned the by-election as a "vanity project" and warned that a Labour candidate would split the vote against Mr Goldsmith.

Writing on the LabourList website, they said: "If there is any chance of kicking Goldsmith out of Parliament, the vote against him must not be split. That's why we think Labour should consider not standing a candidate in this by-election."

The Green Party is still deciding whether to field a candidate.

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A spokesman said: "This by-election could be a real chance to reduce the Conservative majority in Parliament, and replace a candidate disgraced by his racist campaign to be Mayor of London.

"At the core of any agreement must be local decision-making, so the Green Party leadership will be meeting with the local members to discuss their plans, and it will be those members who have a final say.

"Needless to say, any such discussion doesn't discount from the fact that the Green Party maintains distinct positions from both Labour and the Lib Dems - specifically we are the only party to say no to airport expansion across the UK."