Ukip will not contest the Richmond Park by-election in a bid to bolster Zac Goldsmith's support, the party announced today (27 October).
Ukip is urging its supporters to back the former Conservative MP after he quit the House of Commons in protest over the government's decision to support a third runway at Heathrow Airport.
Goldsmith will run as an independent for the West London seat, while the Conservatives have decided not to put up a candidate.
The Liberal Democrats, who came second in the constituency at the general election, are Goldsmith's closest rivals.
The party also opposes the expansion of Heathrow so the EU referendum is expected to be a big issue in the campaign, particularly after a majority of Richmond-upon-Thames residents voted Remain (75,396 to 33,410).
Goldsmith, in contrast, campaigned for a Brexit, while the Liberal Democrats have maintained a pro-EU position.
"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," a Ukip spokesman said.
"Recognising Zac as a principled man, who was fully committed to helping get Britain out of the EU, 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."
The Greens are also considering not contesting the by-election in bid to give the Liberal Democrats a free run to defeat Goldsmith.
But Labour will field a candidate, despite shadow minister Clive Lewis, Lisa Nandy and Jonathan Reynolds warning against the move. The vote is expected in December.
UPDATE: 15:56 BST (27 October)
Ukip donor and Leave.EU co-founder Arron Banks told IBTimes UK his organisation will also back Goldsmith.