Ukip leader Paul Nuttall has denied the party is in crisis after his only MP quit, insisting Douglas Carswell was being investigated over an alleged plot to undermine it.

Mr Carswell’s dramatic decision to resign and sit in the Commons as an Independent – declaring “job done” after the Brexit vote – would make “no difference”, the embattled leader said.

The Clacton MP announced he was walking out because Ukip had “achieved what we set out to achieve” with the nation’s decision to leave the EU.

“On Wednesday, Theresa May will be triggering Article 50 and that was the central purpose of Ukip – to get us out of the European Union. We have won, let’s declare victory with honour,” Mr Carswell told Sky News.

But Mr Nuttall described the move as “not a surprise”, revealing Mr Carswell was due to face an inquiry panel on Monday over evidence he only defected to Ukip in order to undermine Nigel Farage.

The explosive claim – made in a book about the Brexit referendum – alleged Mr Carswell switched to Ukip from the Conservatives to lessen Mr Farage's role, believing it made victory more likely.

Mr Nuttall said: “Douglas was genuinely committed to Brexit, but was never a comfortable Ukipper.

“On Monday, he had been due to meet with the national executive committee to answer various questions relating to longstanding issues, including published allegations that he had joined us in order to try to minimise Nigel Farage’s involvement in the referendum.”

The Ukip leader added: “Our party has not benefited financially or organisationally from having Douglas in Westminster.

“We now have an opportunity to put behind us the most damaging internal conflict which has dogged us over the past year, and look forward with optimism and unity of purpose to the very real challenges of policing Brexit.”

But Opposition parties seized on the resignation to hail the party’s looming demise, Labour’s Andrew Gwynne said: “Ukip are tearing themselves apart. Under Paul Nuttall, Ukip now has no MPs and no future.”

And Tim Farron, the Liberal Democrat leader, said: “Ukip has no purpose. Theresa May is now effectively Ukip's leader and has adopted its hard Brexit agenda.”

The allegation of “infiltration” was made in political journalist Owen Bennett’s highly-praised book The Brexit Club, centering on the “Farage Paradox” – that, as Ukip’s popularity increased, support for leaving the EU fell.

Mr Farage backed the strongly anti-immigrant Leave.EU as the official Brexit campaign, but – with Mr Carswell’s backing – the rival Vote Leave grouping won the all-important designation and funding.

The official Leave.EU Twitter account, responding to his announcement, suggested Mr Carswell was “a Tory plant all along”.

And Mr Farage said: “Carswell has jumped before he was pushed. Whilst he did want to leave the EU, he was never Ukip and sought to undermine us. He should have gone some time ago.”

Mr Carswell denied the allegation, telling Sky News: "The idea that I was involved in sort of subterfuge or some sort of covert operation is nonsense.”

However, the Clacton MP will face demands for him to face his voters by calling a by-election, as he did when he defected from the Tories in 2014.

But Mr Carswell insisted a by-election was unnecessary because he would “not be switching parties, nor crossing the floor to the Conservatives”.

He is rumoured to have held talks with the Tory Chief Whip, but a return would be unpopular with many MPs in his old party.

The loss of Mr Carswell leaves Ukip in the grim position of having no MPs and with little prospect of gaining any, after recent by-election defeats.

Mr Nuttall became leader last year, but lost a by-election in Stoke-on-Trent Central after admitting that claims on his website that he lost close friends at the Hillsborough disaster were untrue.