Ukip's best-known figures have been blocked from standing to replace Nigel Farage under the party's leadership contest rules.
The ruling National Executive Committee (NEC) has said all candidates must have been a member of the party for at least five years.
It means Ukip's only MP, Douglas Carswell, and Suzanne Evans, who is temporarily suspended from the party, will not be able to enter the contest.
Ms Evans said the decision was "ridiculous" and accused the NEC of "changing the rules as it goes along".
She said: "I cannot imagine a decision more ridiculous and more likely to bring Ukip into disrepute than one which means people such as Douglas Carswell and Mark Reckless, our current and former MPs, and the latter also a Welsh AM, cannot stand, despite the huge dedication they have shown to the party and their current elected positions.
"I hope the new leader will allow members to attend NEC meetings unless confidential information is being discussed; publish the minutes; and set up a 'right of recall' so NEC members are accountable to members.
"At the moment, frankly, by operating in secret; changing the rules as it goes along; and being totally unaccountable, it is behaving like the EU."
Candidates must pay a £5,000 deposit to enter the contest, which will be refunded if they secure at least 5% of the vote.
Nominations, which need to be backed by a proposer and 50 supporters from at least 10 branches, opened on Monday and will close on July 31.
During August, regional hustings will be held before the count and declarations are held on September 15.