"Several thousand" of infiltrators - including Conservatives - may have infiltrated the Labour leadership contest, potentially leaving the result open to legal challenge, Andy Burnham's campaign team has warned.
Michael Dugher, Burnham's campaign chief, said not enough was being done to root out infiltrators from other parties as he called for an urgent meeting to discuss the concerns.
In a letter to Labour general secretary Iain McNichol, Mr Dugher said the party was "allowing the issue to drift, and leaving insufficient time for the party to act".
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.
Front-runner Jeremy Corbyn however, told ITV News that the Labour party should be "very happy" about 600,000 people signing up as members and supporters to take part in the vote.
It's great that so many people want to be involved in a process. Let's be happy about democracy, it's thriving.
Labour insists it has a "robust system" to prevent fraudulent or malicious applications.
New rules, introduced by former leader Ed Miliband, allow members of the public to sign up to vote as a "registered supporter" for £3.
But the contest has been plagued by concerns over "entryism" from hard left individuals and Tories seeking to influence the outcome of the race.
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.
A Labour spokesperson said:
We hold regular meetings and calls with candidate teams to update them on the process and will continue to do so.