A senior Labour MP has called for its leadership contest to be "paused" over fears it has been infiltrated by supporters of other parties.
Barry Sheerman said those registering to take part included members of the Socialist Workers Party, the Green Party, the Conservatives and UKIP.
Labour rejects claims of "hard left" and Conservative supporters signing up to back left winger Jeremy Corbyn.
And a Labour spokeswoman rejected calls to suspend the leadership contest.
She said said there was a "very robust" system in place to prevent fraudulent and malicious applications and additional checks are being carried out to make sure the rules are upheld.
An exclusion list is being drawn up of those who have stood against Labour in the past or had helped others to oppose the party. The BBC understands that up to 1,000 applications have been rejected.
One of Mr Corbyn's backers called Mr Sheerman's suggestion "ridiculous".
Backbench MP Diane Abbott said it was coming from people "who think their side will lose".
She added: "This election is being fought under rules that were agreed by the whole party last year."
Under the new rules, introduced by previous leader Ed Miliband, people can sign up as registered supporters for £3 and take part in the vote.
They are asked to confirm they "support the aims and values of the Labour Party".
The popularity of Mr Corbyn's campaign has sparked warnings from the other candidates, Andy Burnham, Yvette Cooper and Liz Kendall, about the party moving to the left.
Mr Sheerman's comments follow similar sentiments from two backbench MPs, Graham Stringer and John Mann, who called for the leadership contest to be halted.
The Huddersfield MP and former select committee chairman, first elected in 1979, told the BBC there were large numbers of people joining Labour to vote for the new leader and some of their reasons were "malign".
Labour MPs were "dumbfounded" when they learned how the contest was to be run and the process was now in "real trouble", he said.
Labour leadership contest
- Who are the candidates? Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall
- Dates: Ballot papers will be sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is announced on 12 September
- Who can vote? All party members, registered supporters and affiliated supporters - including those joining via a union
- What is the voting system? The Alternative Vote system is being used so voters are asked to rank candidates in order of preference
- How does it work? If no candidate gets 50% of all votes cast, the candidate in fourth place is eliminated. Their second preference votes are then redistributed among the remaining three. If there is still no winner, the third place candidate is eliminated with their second preferences (or third in the case of votes transferred from the fourth place candidates) redistributed. It is then a head-to-head between the last two candidates
Labour's acting leader Harriet Harman has emailed the party's MPs with a list of new members in their constituencies asking them to pick out any suspicious names.
Mr Sheerman said although he may be able to pick up some "usual suspects", many others would not be detected.
He denied his concerns were in any way motivated by his support for Liz Kendall, saying he had fought against "entryism" before and believed the current process was flawed.
Labour says the verification process is ongoing, and that ballot papers have not yet been sent out.
Labour recently rejected an application from Conservative MP Tim Loughton, who said he tried to become a registered supporter to highlight flaws in the system.
Speaking on BBC Radio 4's the World at One, Ms Kendall said it was "great" that more people were signing up to Labour and said she had not seen anything that concerned her.
"The important thing is that the checks are put in place," she added.
The Trade Unionist and Socialist Coalition, which stood in 135 constituencies in May's general election and includes the Socialist Workers Party, said reports of infiltration were down to "Blairite manoeuvres" against the Corbyn campaign.
It said one of its members, former councillor Chris Flood, had been wrongly named in a report in the Times as having signed up.
Labour's former communications director, Alastair Campbell, said the party "could be finished" if Mr Corbyn wins.
On his blog, he warned of a "car crash" scenario and urged supporters to back "anyone but Corbyn", warning it would not be easy to replace him if he won and Labour was struggling in the polls.