Labour has been criticised for saying it will cancel the leadership election votes of supporters after they have been cast.
Party officials have said they will continue their drive to root out supporters of other groups, even after voting has begun this weekend.
But there are worries the approach could leave some room for foul play – especially with an insurgent candidate opposed by many in the party’s establishment set to do well.
Darren Hughes, deputy chief executive of the Electoral Reform Society, which campaigns for a fairer voting system, said the ballot should be delayed instead of retroactively cancelling vote.
“Instead of getting things started and then destroying some votes later on if they find people from other parties, Labour should delay sending out the ballots for a few more days so they can sort this out before it gets worse,” he said.
“It was always going to be a heroic undertaking to deal with these hundreds of thousands of new members and supporters in such a short space of time. The best thing to do now would be to spend a little more time checking who can vote before ballots go out to ensure there is faith in the democratic process.”
Labour insider had told the Daily Mirror newspaper that the vetting process would continue.
"No ballot paper will go to someone who's not been verified, but the verification process goes on," a source said.
"We have big lists and we're constantly improving and updating those lists, and things may emerge once someone's received their ballot.
"We can then cancel their ballots. ERS will anonymise them but will be able to refer to the number that's been generated and remove them."
Labour leadership: The Contenders
Labour leadership: The Contenders
1/4 Jeremy Corbyn
Jeremy Corbyn readily admits he is only standing to ensure the left of the party is given a voice in a contest dominated by candidates promising to move the party towards the centre-ground of British politics
Profiles by Matt Dathan
2/4 Andy Burnham
Andy Burnham is the current front-runner to win the leadership election according to bookmakers, but the fact that the Conservative party leadership hopes he wins shows the task that awaits if he is Ed Miliband’s successor. He will have to find a way of distancing himself from both the last five years under Mr Miliband and the Blair and Brown years, during which he served in the Cabinet
3/4 Yvette Cooper
Yvette Cooper will also face a battle in convincing voters she offers a sufficient break with the past, having served in Gordon Brown’s Cabinet and she played a key role in Mr Miliband’s team as shadow home secretary. The fact that her husband is Ed Balls will not have a negative impact internally but voters are not likely to look favourably on the prospect of Mr Miliband’s ousted shadow chancellor entering Downing Street if Ms Cooper wins in 2020
4/4 Liz Kendall
Liz Kendall faces criticism over her lack of experience – she was only elected in 2010 and has no experience of serving in government and wasn't even in Ed Miliband’s shadow cabinet. But that very lack of experience means she can make a pitch as the only candidate offering real change and a real break from the Blair/Brown/Miliband years
Mr Hughes added that there were “clearly problems” with the election process and that the checking was necessary.
“Faith in the integrity of the ballot is absolutely vital for a democratic process, so being transparent and honest is crucial,” he added.
The ERS is separate from Electoral Reform Services, a commercial venture related to the society that will be administering the ballot and providing technical expertise to ensure the vote is properly conducted.
The deadline to register to vote in the Labour leadership election has already passed. Ballot papers and voting are due to begin this weekend, with the result announced at a special conference in September.