Labour leadership frontrunner Jeremy Corbyn has dismissed claims that the contest is being skewed by infiltrators as "nonsense".
The left-winger hit out ahead of a crunch meeting between all four candidates and acting leader Harriet Harman to discuss the issues.
Speaking at a BBC Radio 5 Live hustings, Mr Corbyn questioned whether there were any Tory infiltrators and suggested others were being unfairly turned away.
"Are there any Tory infiltrators? I think there has been a lot of nonsense in the papers. 600,000 people have either joined the Labour Party or signed up as supporters, we should be pleased about that," he said.
"There are a few Tory MPs I understand tried to register, got rejected. End of story."
Shadow health secretary Andy Burnham said he believed there was a problem, but confirmed that he would not attempt to challenge the result in court even if he was narrowly defeated.
"I wouldn't want to overstate this whole issue, but there is some evidence that Tories are signed up to vote," he told the audience in Stevenage.
"I was in a meeting in Milton Keynes on Sunday when one stood up in the audience and said he had voted in our contest.
"It is for the party to decide. All we want to ensure is that they have been properly implemented and all the information the Labour Party has about these things has been properly used.
"I don't think there is any great big deal. We just want to clarify the issue so when we get to September 12 we can move forward together."
Asked directly whether he ruled out a legal challenge, Mr Burnham said: "Under all circumstances absolutely. After this the Labour has had a vibrant debate, a good debate. But by God, let's move forward and attack the real enemy, the Conservative Party."