UKIP financial backer Arron Banks claims he has been suspended from the party.
Banks gave £1 million to UKIP in the 2015 General Election and also funded the Leave.EU campaign in 2016.
The tycoon – who had been involved in a public spat with the party’s only MP, Douglas Carswell, and was critical of party leader Paul Nuttall - claims his membership had lapsed.
He says that when he reapplied, he was told he’d been suspended until an appearance in front of the party's National Executive Committee.
He released this comment on Twitter: “UKIP has somehow allowed my membership to lapse this year, despite me giving considerably more than the annual membership fee over the past year.
"On reapplying I was told my membership was suspended pending my appearance at an NEC meeting.
"Apparently, my comments about the party being run like a squash club committee and Mr Carswell have not gone down well.
"I now realise I was being unfair to squash clubs all over the UK and I apologise to them.
The party itself, however, have dismissed this, saying Banks wasn’t a member of the party from the beginning of 2017.
A UKIP spokesman said: "Mr Banks was informed that his membership was due for renewal in October 2016.
“He chose not to renew, despite reminders. Consequently, his membership expired on January 1 2017.
"It isn’t possible to suspend Mr Banks from a party where he isn’t a member."
The news throws UKIP into fresh crisis, after a difficult few months for the eurosceptic party.
Although UKIP achieved a landmark success in last year's EU referendum, and figurehead Nigel Farage successfully aligned himself with Donald Trump during the US elections, the party was subject to ridicule after Diane James quit as leader after just 18 days last autumn.
This was followed by a fight between two MEPs in the European Parliament, which resulted in leadership favourite Steven Woolfe being hospitalised.
Last month the party's new leader, Paul Nuttall, was forced to deny earlier claims that he had lost friends at the 1989 Hillsborough stadium disaster, and then failed to win the by-election in Stoke, a city which has been dubbed Britain's 'Brexit capital.'