Andy Burnham's campaign has warned too little is being done by Labour to root out infiltrators in the leadership election, leaving the result open to legal challenge.
A letter calling for an urgent meeting to discuss fears that "several thousand" Conservatives and others have signed up for a vote has been sent by Mr Burnham's campaign chief Michael Dugher.
The contest has been marred by claims of "entryism" by political opponents paying £3 to become registered Labour supporters under new party rules - giving them a vote in the poll.
More than 120,000 people have signed up, along with 189,000-plus members of unions and other affiliates, swelling the electorate to more than 600,000.
The party has begun blocking people it does not believe support the aims of the party, sparking a backlash from some Jeremy Corbyn supporters who claim it is being rigged.
In a letter to Labour general secretary Iain McNichol, Mr Dugher said the party was "allowing the issue to drift, and potentially leaving insufficient time for the party to act".
There is "potentially more that can be done by the party" to weed out Tories and other rogue voters," he said, but no discussion with candidates is planned until September 1.
"We are also concerned that given the party's limited resources and the effort required to investigate applicants, this could result in the integrity of the contest being called into question, and the outcome subject to legal challenge," he said.
It came as Yvette Cooper was accusing Labour colleagues of "defeatism" and insisting she can still pull off an opinion poll-defying victory over Mr Corbyn in the leadership race.
In an appeal to wavering voters not to "write our party off", amid widespread expectations of a win for the veteran left-winger, she will say he is showing signs of "hubris".
Mr Corbyn has said that if elected on September 12, he would make a formal apology for the party taking the UK into the Iraq War "on the basis of deception".
The anti-war campaigner - who voted against Tony Blair's decision to join the US-led action - said he would also say sorry to the Iraqi people for "the suffering we helped cause".
After starting the contest as a rank outsider, Mr Corbyn has become the overwhelming favourite to succeed Ed Miliband, enjoying significant leads in many opinion polls.
It has left MPs from other wings of the party planning how to respond, with one claiming plots to oust him would begin immediately after he took the reins.
Ms Cooper will tell party activists in Devon that the result is not a "done deal".
"I fear for Labour's future if we get this wrong. But now is not the time to give up and plan for what comes next when our party has the choice in its hands," she will say.
"Don't write our party off based on some close polls. Surely May 8 taught us a bit of scepticism about poll results.
"Don't think it's a done deal because of the bookies' odds. Paddy Power hasn't got a hotline to the hearts of Labour Party members
"Hundreds of thousands of people are yet to even open their ballot papers. Many thousands are still undecided.
"This is no time for hubris from those who think they are winning. And no time for defeatism from those who believe we need another way.
"No time to stand back and let our party drift into another election defeat."
She said Mr Corbyn was offering "a fantasy - easy to sell but impossible to realise".