Harriet Harman has insisted there will be no doubt about the "integrity" of the Labour leadership contest as it emerged that less than 1% of new supporters have been blocked as 'infiltrators'.
Speaking after a meeting with the four candidates to discuss concerns, the acting party leader said every effort was being made to weed out those who were trying to skew the result.
But figures released by Labour showed that just 3,100 people have been barred from voting because they do not share its "aims and values".
That compares to 366,000 who have signed up as full members, affiliate supporters, or paid £3 to become registered supporters since the general election.
The total electorate is now expected to be around 554,000, significantly lower than the 610,000 previously estimated. However, the reduction is mostly due to 15% of applicants being rejected because they are not on the electoral roll.
Many affiliates garnered from unions have also been ruled out because they were already due to vote as party members.
Mrs Harman defended her handling of the process, which has seen policy debates largely overshadowed by fears political opponents are manipulating the system to ensure victory for left-winger Jeremy Corbyn.
She said: "Those people who don't support the aims and values of the Labour Party are not entitled to vote and we will continue the process of verification, of making sure that those who do not support our aims and values but are trying to vote - trying to cheat their way into the system - that they are identified and their vote is cancelled.
"That will carry on right up until the last minute."
Mrs Harman said she did not think any of the candidates were criticising her handling of the contest.
"No, they are not criticising the way I have handled it," she said. "I think they are recognising that I am going about it with an absolute due diligence to implement the 2014 constitutional arrangements."
At a BBC Radio 5 Live hustings in the town earlier, bookies' favourite Mr Corbyn branded claims the race was being fixed by infiltrators "nonsense".
"Are there any Tory infiltrators? I think there has been a lot of nonsense in the papers...
"There are a few Tory MPs I understand tried to register, got rejected. End of story."
Shadow health secretary Andy Burnham suggested 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 Labour has had a vibrant debate, a good debate. But by God, let's move forward and attack the real enemy, the Conservative Party."
Aides to Mr Burnham said he had found the meeting with Mrs Harman "reassuring".
Shadow home secretary Yvette Cooper said: "We shouldn't be drawing up the rules for this election. That should be a matter for the Labour Party, they have got to make sure that proper robust checks are in place.
"And we must not get distracted into just talking about process when there are so many big ideas, so many people still left to vote."
Shadow health minister Liz Kendall, who admitted she was "quite a long way behind" in the race, said she thought the numbers of 'infiltrators' who would be voting was a "tiny minority".
Labour later released new figures for the electorate in the leadership race. The total number eligible to vote is now 553,954, including 292,973 full party members - nearly 106,000 more than at the general election. There are 148,182 affiliated supporters and 112,799 registered supporters.
According to the party, all applications to take part in the ballot have now been "administratively processed" - checked against the electoral register.
Some 15% were struck off because they were not on the electoral register.
Enquiries to ensure applicants share Labour's "aims and values" are being carried out round the clock and seven days a week by 70 staff in Newcastle, more than 30 staff in London and more than 30 elsewhere in the UK.
Questionable cases are sent to a panel of elected members of the National Executive Committee (NEC) for a decision on whether they are eligible.
Of 3,502 considered by the panel, 3,138 have been ruled ineligible - including 1,972 registered supporters who paid £3 to join the vote.
Some 400 applicants were found to be Tory members or supporters, and 1,900 members or supporters of the Green Party.
Sources said the bulk of supporters had now been assessed to see whether they need to go before a panel, and the numbers ruled ineligible are not expected to rise significantly.
The party said its deputy returning officer and independent legal adviser, John Sharpe, had given an opinion that the process was "robust" and would be compliant with the rules introduced in 2014.
During the latest hustings debate, Ms Cooper and Mr Corbyn clashed over his plans effectively to print more money in order to fund higher government spending.
"What he is talking about is printing money when the economy is growing ... it is dodgy economics and it would be ripped apart," she said.
"It would just push up inflation, it would make us all worse off, it would create a cost-of-living crisis and it is printing money we haven't got."
But the left-winger insisted Labour's problem at the general election had been that it was "not offering an alternative to austerity".
"£325 billion was put into the banks as quantitative easing as essentially a kind of loan during the banking crisis," he said.
"My suggestion is that we need serious investment in infrastructure, not through expensive private finance initiative but through public investment, and I am suggesting a people's quantitative easing which will provide that necessary finance to kick-start the development of the economy - not just in London and the South East, but in the North and Scotland as well."
Mr Burnham triggered murmurs of protest after suggesting Labour should have a woman leader only "when the time was right".
He quickly clarified that he meant when the "right candidate" came along.
Mr Burnham also indicated that he would not serve on the Labour frontbench if the party's policy was to scrap Trident or leave Nato - positions backed by Mr Corbyn.
"Those would not be policies that I could support. I would not support a policy to leave Nato," he said.