Ukip has backed Zac Goldsmith in the Richmond Park by-election in a move that could mark him out as the Brexit candidate in a seat the Liberal Democrats think they can win on a pro-EU ticket.
Interim Ukip leader Nigel Farage and the party's ruling executive committee have joined the Tories in deciding not to field a candidate in the December 1 election.
Lib Dem leader Tim Farron said the endorsement showed Mr Goldsmith is a "hard Brexiteer" who wants Britain out of the European single market.
Pro-Leave Mr Goldsmith will fight the election as an independent following his decision to quit as a Tory MP in protest at the Government's support for a third runway at Heathrow Airport.
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 they have been heartened by a strong performance in David Cameron's former constituency of Witney last week, and Richmond Park's 69% Remain vote in the referendum.
Mr Goldsmith will attempt to frame the election as a protest against Heathrow expansion but the endorsement of Mr Farage could favour the Lib Dems' aim of turning it into a referendum on Brexit.
A Ukip spokesman said: "Zac Goldsmith has resigned on a matter of principle and Ukip admire him for having the courage to do so. Ukip have always believed that Gatwick was a preferred option to Heathrow.
"Recognising Zac as a principled man, who was fully committed to helping get Britain out of the European Union, Ukip leader Nigel Farage, in conjunction with our National Executive Committee have agreed that we will not be fielding a candidate in the upcoming by-election for Richmond Park.
"The Liberal Democrats are committed to overturning the result of the 2016 referendum and ignoring the will of the British people. This must not be allowed to happen.
"Ukip are encouraging all of our supporters and voters to support Zac Goldsmith in his bid to become an independent MP."
Mr Farron said Mr Goldsmith faces a "major battle" with the Lib Dems.
He said: "Zac Goldsmith claimed Brexit has nothing to do with this by-election. The very public endorsement he has picked up from the party of Nigel Farage nails that lie.
"Zac Goldsmith was already the Conservative Party candidate. Now he is also the Ukip candidate. His campaign is the Nigel and Zac show.
"Ukip's website expressly states that it is vital for supporters of hard-Brexit to defeat the Liberal Democrats in Richmond Park, and praises Zac Goldsmith for being 'fully committed to getting Britain out of the European Union'.
"This by-election presents a golden opportunity to defeat one of the leading Brexiteers who is determined to even take Britain out of the single market.
"He might be able to afford the huge damage this would do to our economy, but many people in Richmond Park are worried about the effect on their jobs and livelihoods as a result of the Conservative government playing Russian roulette with the British economy."