Ukip members will decide whether Henry Bolton has a future as leader of the crisis-hit party.
Activists will attend the emergency meeting to decide whether to endorse a motion of no confidence in his leadership passed by the party’s ruling national executive committee (NEC).
But Ukip’s general secretary suggested that Mr Bolton’s days as leader are numbered even if he survives the vote.
Paul Oakley said “Henry will be out after the local elections if he isn’t out today” as the party was not prepared for May’s contests.
Mr Bolton is clinging on to the leadership despite a revolt by several senior figures in the party in protest at his relationship with model Jo Marney.
Mr Bolton said he still has “strong affections” for his former girlfriend who sent offensive messages about Meghan Markle, and has compared the scrutiny of his private life to that faced by Diana, Princess of Wales.
Paul Oakley, pictured with Nigel Farage, hit out at Henry Bolton’s leadership (Victoria Jones/PA)
Mr Oakley told BBC Radio 4’s Today programme: “I have no interest in his personal life, it’s basically, for me, the fact that he has done nothing as leader. He is not making the case for Brexit.
“A particular problem is we have got local elections in May – this time in 2014 we were already well down the campaigning route and we have done nothing.”
The row around Ms Marney’s messages was “the last straw” and Mr Bolton’s handling of the issue indicates “he just doesn’t get it”.
He said the party was already having discussions about an interim leader to replace Mr Bolton and there are “three people in the frame”.
Mr Bolton hit out at the media interest following the breakdown of his marriage to Tatiana Smurova and his new relationship with Ms Marney.
He told Sky News: “The only people that are truly affected by that are my wife and children, and I feel deeply, deeply sad for what pain and hurt I’ve caused them but it’s a private matter and it needs to be dealt with privately.
“I understand the scrutiny and public interest but it’s been going on for six weeks.
“There’s probably been nothing like that since the Profumo affair or Diana and Dodi (Fayed).”
The extraordinary general meeting of the party in Birmingham will decide whether or not to endorse the NEC’s no confidence vote.
But Mr Bolton “does not rule out” running for leader again in the event of losing on Saturday.
Mr Bolton, who has called for sweeping changes to the way the party is run, claimed he was the victim of rivals “stirring the pot”.
“The people who started complaining about this were people on the NEC, none of whom voted for me in the leadership election,” he said.
“Secondly, there were a small number of individuals within the party, failed leadership contenders, who saw an opportunity in me leaving my wife to go in for the kill – they’re stirring the pot… if they were truly dedicated to the party and their country and not to their own they would have stayed at their posts.”
The cash-strapped party faces a series of challenges, including the prospect of a hefty legal bill over a defamation action brought by Rotherham’s three Labour MPs against MEP Jane Collins.
Nigel Farage has offered some support to Henry Bolton (Philip Toscano/PA)
Former leader Nigel Farage handed Mr Bolton a lifeline by supporting him, but said the party was “collapsing”.
Writing in the Daily Telegraph, he said: “I believe it would be better to allow Mr Bolton, with all his faults, the chance to turn Ukip into an electoral machine again.
“The alternative is for the party to carry on down the path of self-destruction into irrelevance. It may be too late to save Ukip, but you never know.”