Ukip is facing a "fight for survival" after popular leadership hopeful Steven Woolfe was excluded from the race to replace Nigel Farage, it has been claimed.
Three members of the party's ruling board immediately quit in protest, claiming the migration spokesman had been deliberately blocked from standing on a technicality.
Mr Woolfe said he was "extremely disappointed" by the move and branded the National Executive Committee (NEC) "not fit for purpose".
The decision plunged the party into a fresh round of bitter in-fighting and fuelled claims that it would split.
Supporters claimed Mr Woolfe had been the victim of a coup led by Ukip's only MP, Douglas Carswell, and Neil Hamilton, group leader in the Welsh Assembly.
The MEP had been the favourite to take the top job but was 17 minutes late submitting his nomination papers, blaming technical difficulties for the delay.
Members of the NEC said that meant his application was "ineligible" and voted to block his candidacy by a "clear majority".
"I am extremely disappointed by the Ukip NEC decision to exclude me from the party's leadership election," Mr Woolfe said.
"Having been a committed member of Ukip, standing for the party in multiple elections, acting as a spokesman at the highest level, I wanted to take this opportunity to stand for leader to inject my ideas, plans and passion into the party.
"Over the course of this leadership election, the NEC has proven it is not fit for purpose and it confirmed many members' fears that it is neither effective nor professional in the way it governs the party."
Michael McGough, Victoria Ayling and MEP Raymond Finch announced they were quitting the NEC over the "deliberate obstruction" of Mr Woolfe's nomination.
The board "has essentially usurped full governance of the party" and is "collectively in pursuit of oligarchy, self-promotion and cronyism", they said.
"Rather than acting as servants of the party, many on the NEC are acting as the owners. The factionalism has seen some of the party's top talent and most loyal officers wilfully excluded."
"Steven Woolfe is a popular candidate among Ukip's members and should be permitted to represent those that wish to vote for him," they added.
"To purposefully trawl for technicalities upon which to base a decision to deny his inclusion is not in the best interests of the membership and truly injurious to Ukip."
Mr McGough said the party "could be finished" and suggested Mr Carswell and suspended party member Suzanne Evans were behind the move, alongside other centrists.
He told BBC Radio 4's World At One programme: "To me this is a fight for survival.
"If we don't get this right, we don't get a competent leader who's comfortable with the media, the party could be finished.
"It's life or death now and that's why I've taken such a strong decision to leave the NEC.
"Because we need reform and we need it now so that we are ready to fight the next general election and the local elections next year where we're supposed to do quite well."
Ukip party chairman Paul Oakden denied the move to bar Mr Woolfe from the contest was petty and said the rules have to be followed.
"Will the party split? I hope not, I don't think so," he told the programme.
London Assembly Member Peter Whittle said Mr Woolfe was an "exceptional candidate", and attacked the NEC.
"I am outraged at the NEC's decision to wrongfully keep Steven Woolfe off the ballot," he said.
"This obscure and unworthy body of people who are totally unfit for the position of making such decisions has, in my experience, done nothing but thwart the onward progress of Ukip and indeed seem to apparently resent those with real talent and electoral success.
"It is time for radical change within Ukip and that change needs to happen now."
The exclusion of Mr Woolfe from the contest paves the way for home affairs spokesman Diane James to succeed Mr Farage, with bookies cutting her odds of winning.
MEPs Bill Etheridge and Jonathan Arnott along with Elizabeth Jones, Councillor Lisa Duffy and Phillip Broughton will all be on the ballot paper.
A Ukip spokesman said: "By a clear majority of NEC members, Steven Woolfe MEP's application was considered to be ineligible as a result of a late submission and as such he did not meet the eligibility criteria. His membership of the party was not in question."