The Lib Dem leader, Tim Farron, said the endorsement showed Goldsmith was “the Ukip candidate”.

“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’s already the Ukip candidate. His campaign is the Nigel and Zac show.

“Ukip’s website expressly states that 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.

“Zac Goldsmith can get as many hard-right candidates to give him a clear run as he likes: he knows he faces a major battle with a Liberal Democrat party determined to keep Britain open, tolerant and united.”