Labour's leadership contest is being run rigorously and the result "will stand", acting leader Harriet Harman has insisted amid claims too little is being done by the party to root out infiltrators.
Andy Burnham's campaign has warned the outcome faces being open to legal challenge because thousands of Conservatives may have signed up to vote while supporters of frontrunner Jeremy Corbyn claim the race is being rigged after being blocked from voting.
But Ms Harman insisted the election has been run with constant legal advice and all Labour supporters will get a vote.
She told the BBC: "Because this is the first time we have operated these new rules for electing a Labour leader we have acted constantly on legal advice, we have taken legal advice every step of the way and I am absolutely certain that no court would decide that we had done anything other than apply the rules in a rigorous, fair, robust and even-handed way,"
"So whoever is elected they will be legally elected and that the result will stand."
She added: "People who do support the Labour Party will be eligible for a vote and will get their vote. But there are some people who are saying 'well, I support the Tory Party but I am going to get a vote in Labour's leadership election'. We don't think that there is anything clever or funny about that. That is dishonest and that is shameful for people who purportedly believe in democracy and support democracy."
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.
Mr Burnham's campaign chief Michael Dugher wrote to Labour general secretary Iain McNichol warning that 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.
"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 added.
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.
Contender Yvette Cooper accused Labour colleagues of "defeatism" and insisted she can still pull off an opinion poll-defying victory over Mr Corbyn in the leadership race.
A spokesman for her campaign said the shadow home secretary was "focusing on winning arguments and votes not legal challenges".
Mr Corbyn's campaign said it had confidence in the way the election was being managed.
A spokesman said: "The purely internal procedural obsession falls short of the outward debate the party needs. Whilst some issues have been raised, we do have confidence in management of the process by elected members of Labour's NEC and the general secretary."