UKIP's biggest donor has threatened to withdraw funding unless the party "cleanses" itself of "factions".
Businessman Arron Banks said he was "struggling" to support any of the four contenders vying to replace Nigel Farage as leader.
He also called UKIP's one MP, Douglas Carswell, a "pain in the backside" and urged him to rejoin the Conservatives.
Mr Banks also hinted that Mr Farage could return, calling him the "major politician of the last 20 years".
UKIP has been beset by reports of internal struggles in recent months and Mr Banks, a supporter of Mr Farage, told BBC Radio 4's Today programme his impending departure left the party at a "crossroads".
He said: "Nigel was probably the major politician of the last 20 years. I think losing him has caused the pressure cooker to explode. We are struggling at the moment - there's no point denying it."
Suzanne Evans, Paul Nuttall, John Rees-Evans and Peter Whittle have entered the contest to replace Mr Farage. But Mr Banks, who made his fortune in the insurance business, said: "At the moment I'm struggling with he candidates on offer."
He added: "My problem with all the candidates is they seem to want to push it back to the centre. That's a ludicrous place for it to be."
Questioned on BBC Breakfast as to whether he would continue funding UKIP, Mr Banks replied: "We'll have to wait to see who gets elected. At the moment it's in a hell of a mess. There's no sugar-coating that. But actually the opportunities have never been greater."
Mr Nuttall, a former deputy leader of the party, is currently listed by bookmakers as favourite to win the contest, with the result being announced on 28 November.
Mr Banks, who said he had given UKIP £1m, added: "I like Paul Nuttall a lot. I doubt whether he has the strength to deal with some of the career politicians that have come into the party.
"You couldn't accuse Nigel of being a career politician. He didn't really want to become a politician. He had one issue that he wanted to fight, and I think whoever becomes leader has to find their issue and the thing they want to campaign on."
Asked whether he would continue funding UKIP, he replied: "At the moment I'm sitting on the fence. Unless the party cleanses itself of some of the factions that have caused it so much trouble, I won't be doing anything, but we have to wait and see."
He criticised Douglas Carswell, who defected from the Conservatives in 2014 and has since become involved in disagreements with Mr Farage.
Mr Banks said: "Mr Carswell has been the biggest pain in the backside... I wish he would go back. They won't have him back."
He added: "There has to be a clearout of certain people in the party who have caused a hell of a lot of problems."
Mr Banks also suggested Mr Farage could return to the leadership at some stage, saying: "I think at some point he may do. I think he's gone on record as saying that if [Theresa] May doesn't deliver on Brexit he will come back. In politics you can never say never, can you?"
Mr Farage is filling the leader's role on an interim basis after his successor, Diane James, resigned only 18 days into the job.