Harriet Harman will seek to reassure the Labour leadership candidates they face a fair contest amid claims of mass infiltration by political opponents.
The interim leader has invited the four rivals for talks on Tuesday to address warnings inadequate vetting would leave the result of the race open to legal challenge.
The contest has been marred by claims of "entryism" by Conservatives and others paying £3 to become registered Labour supporters under new party rules - giving them a vote in the poll.
There have also been complaints about legitimate voters - many of them supporters of surprise front-runner Jeremy Corbyn - being unfairly blocked.
In a fresh sign of the level of concern felt by senior party figures about the growing prospects of a win for Mr Corbyn on September 12, Gordon Brown publicly backed Yvette Cooper.
The former prime minister's office announced that he had given his second-preference vote to Andy Burnham and his third to Liz Kendall, a clearly calculated snub to the veteran left-winger.
Mr Brown had not endorsed any of the rivals when he recently delivered a thinly veiled warning to the party not to elect Mr Corbyn if it wanted to win the next general election.
Andy Burnham, whose campaign demanded the emergency meeting, said he needed to be "more assured" about the way the contest was being run but ruled out launching any legal challenge.
"I have confidence in it as long as we get clarification about how the very small number of applications that have been made on a fraudulent basis (are) being dealt with properly," he said.
The shadow health secretary warned against the party "turning inward and talking to ourselves" amid turmoil within the ranks of MPs over the possibility of Mr Corbyn taking the reins.
He told supporters at a rally on Monday night that he was determined not to allow the Labour Party to split - comparing it with reports about the imminent demise of a well-known boy band.
"Have you heard the big news today? One Direction are splitting up next year. I'm here to tell you, I'm not going to let the same thing happen to the Labour Party," he told a rally of supporters.
"We are here to get a Labour government back. We're here to win.
"The Labour Party I lead will be the most hard-working opposition that anyone has ever seen, we will run the Government ragged every day.
"And we will be an opposition fighting not for its own causes, but one that is fighting to win power."
Ms Cooper promised a number of reforms to attract a wider cross-section of the public to enter politics - including appointing a shadow minister for young people and a "diversity champion".
With Mr Corbyn credited with re-engaging large numbers of younger voters to the party, she promised to give youth and student supporters more opportunities.
He pledged to create a fund to help working-class activists afford to seek nomination as general election candidates, warning too many Labour MPs were privately educated professional politicians.
Former home secretary Charles Clarke branded the contest "a disaster" which would probably be the subject of multiple legal challenges.
"I think it's been a disaster actually, I'm very sad about it," he told BBC2's Newsnight.
"We've got legal challenges, I think there may still be further legal challenges about the process, issues about who can vote who can't vote, many party members who have been active for years distressed to see people who have been campaigning against the party with an equal vote to them in these circumstances, and I think we've made a series of mistakes.
Asked who was to blame, he said: "I think the National Executive Committee, I don't think Harriet's done it very well, I think the changes Ed Miliband brought in were a mistake and many people argued that at the time, and there have been a series of mistakes of this kind."
Mr Clarke said he had voted for Ms Kendall, ahead of Ms Cooper and Mr Burnham although he suggested the line-up lacked a single outstanding candidate.
All the hopefuls were "worse than some previous leaders we've had".
"What I worry about is that we really have to summon ourselves up and say 'OK, how are we going to move out of this to a situation where Labour can regain the confidence of people in the country.
"I don't think that Jeremy will be able to do that. I do think the other three have a chance of doing that."