One of Ukip's major donors, Arron Banks, has threatened to pull his funding unless he is made chairman so he can stop the party being "run like a jumble sale".
Following party leader Paul Nuttall's defeat in the Stoke-on-Trent Central by-election, Mr Banks blamed "dullards" like Ukip's only MP Douglas Carswell for not bringing in enough Tory votes.
He said the defeat was the "final straw" for Mr Carswell, accusing him of being preoccupied with "sabotaging" a knighthood for former leader Nigel Farage, an accusation the MP denies.
Mr Banks said Mr Farage would have a "more fulsome role" in the party under his stewardship.
Mr Banks told The Sunday Express: "I am giving Paul Nuttall an ultimatum that either I become chairman and sort out Ukip by bringing in business people and professionals to make the party electable, or I am out of there.
"The party cannot continue to be run like a jumble sale.
"If Nuttall doesn't professionalise it and toss out the likes of Douglas Carswell, Suzanne Evans and the rest of the Tory cabal then the party is finished anyway."
He continued: "These dullards aren't bringing in Tory votes, Stoke proved that, so what are they for?
"The party now needs to bring in serious people to fix its ramshackle administration, stay relevant, and stay radical or it will die.
"From what I hear far from winning elections, Carswell's main concern is sabotaging Nigel's knighthood, which is why he must leave the party."
Mr Banks said he will make a formal complaint about Mr Carswell to the party's national executive committee for bringing the party into "disrepute".
Ukip had seen Stoke, which voted for Brexit in the referendum last year, as fertile ground to challenge Labour, but Mr Nuttall's gamble of standing himself failed as Labour's Gareth Snell held the seat.