The Liberal Democrats are hoping to turn Richmond Park and North Kingston yellow by focusing on Brexit in the upcoming by-election prompted by Zac Goldsmith's resignation over Heathrow expansion.

Mr Goldsmith has forced the contest with his announcement he is quitting the Conservatives to stand as an independent candidate in protest at the decision to give the green light for a third runway at the airport.

The Lib Dems are set to pour activists into the area in a bid to pull off a spectacular win, with leader Tim Farron determined to make the by-election a vote on Theresa May's "hard Brexit" strategy.

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

However, the Lib Dems have some momentum after the Witney by-election where they slashed the Conservative majority in David Cameron's old seat from 25,000 to 5,700.

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

The constituency recorded one of the biggest Remain votes – 69 per cent - in June's referendum, while Mr Goldsmith was among the leading Tories 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.

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 current strategy is.

He said: “This will be a referendum on Theresa May's leadership of the Conservative Brexit government.

"In Witney we had to spend the first couple of weeks establishing ourselves as the main challenger - here people will see from the get-go that we are the only challenger to the Conservative Brexit government, and we are determined to put in an extremely strong challenge for this seat.

"Our determination to keep Britain open, tolerant and united will, I believe, strike a chord with the people of Richmond Park."

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

Mr Farron said: "I am confident the people of Richmond Park will see this for what it is - a stage-managed, confected by-election by the Conservatives.

"We will have the bizarre spectacle of Zac Goldsmith supposedly running against his own government, but as the only Conservative on the ticket.

"Goldsmith cannot credibly run as the protest vote candidate. He is the candidate of the Conservative Brexit establishment.

"People know that if they vote for Goldsmith he will have made merely a token protest for form's sake. Meanwhile Heathrow and Brexit will carry on full-steam ahead.

"If people vote Lib Dem then that will be taken as a genuine vote against Heathrow and against Brexit."

Despite some Labour MPs calling on their party not to fight the seat to give the Lib Dems a clear run at ousting Mr Goldsmith, it has been confirmed there will be a Labour candidate in the by-election.