Douglas Carswell is facing calls to trigger a by-election after quitting as Ukip's only MP and plunging the party into fresh turmoil.
The Brexit-supporting MP insisted he does not need to call a fresh vote because he is not rejoining the Conservatives or switching allegiances to another party.
But Ukip's biggest financial backer, Arron Banks, has urged the MP to call a by-election in his Clacton constituency so he can stand against him.
The insurance tycoon drew attention to a November 2015 blog post by Mr Carswell, entitled "how to resign and spark a tricky by-election - my guide for rebellious MPs".
Mr Carswell, who defected to Ukip from the Tories in 2014, told Labour MPs exasperated with Jeremy Corbyn's leadership and thinking of jumping ship that they should "insist on a by-election" if they quit.
"Between 1701 and 1918, a by-election had to be called every time an MP was invited to join the government," he wrote.
"Think of it as a sort of confirmation hearing. Insist on a by-election to confirm your move with the electorate."
Mr Banks said the people of Clacton deserve a say on Mr Carswell's future as their MP, claiming he acted in self-interest when he defected from the Tories.
"Douglas Carswell's departure is a rare piece of good news for Ukip," Mr Banks said.
"Carswell has been nothing but a source of division for the party since he joined – a point which Nigel (Farage) and I have stressed constantly for months now.
"His resignation is a good first step, but Ukip will remain in a state of constant dysfunction until the rest of Carswell's Tory cabal follows him out of the door."
It is unclear whether Mr Banks would be able to fight an election under Ukip's banner as he is no longer a member of the party, and said earlier this month that he would concentrate on a new political movement.
Mr Carswell's resigned on Saturday and suggested the party no longer needs to exist after the Brexit vote.
Amid claims that he only defected from the Tories as part of a plot to marginalise Nigel Farage's role in referendum, Mr Carswell admitted he wanted the Brexit campaign to be more "positive", apparently referring to the former leader's controversial views on immigration.
He also revealed he had not spoken to Mr Farage since around December and urged him to be more positive "about the fact that on Wednesday, we win" - referring to the Theresa May's intention to invoke Article 50 to begin the UK's withdrawal from the EU on March 29.
Mr Carswell's departure follows a long-running feud with the former leader, who claimed the MP was "never Ukip" and had "sought to undermine us".
Ukip leader Paul Nuttall, who reportedly only found out about Mr Carswell's resignation via a text from a friend, said the party has not benefited "financially or organisationally" from the MP's presence in Westminster and so his departure would make "no difference" to his reform agenda.
Labour and the Liberal Democrats seized on the row, both declaring that Ukip has no purpose and no future.