Harriet Harman has been urged to suspend the Labour leadership race amid fears it is being warped by a wave of hard-left “infiltrators”.
There is speculation that 140,000 more activists could be eligible to vote than before the general election - with many of them signing up just to back Jeremy Corbyn.
The Communist Party of Great Britain is among the groups that have urged supporters to join Labour and endorse the Islington North MP. Under new rules, they can pay just £3 and take part in the ballot.
Backbencher John Mann told the Sunday Times the contest was “totally out of control”, and insisted acting leader Mrs Harman should step in so that proper checks can be conducted.
“It should be halted,” he said. “It is becoming a farce with long-standing members ... in danger of getting trumped by people who have opposed the Labour Party and want to break it up, expressly want to break it up - some of it is the Militant Tendency types coming back in.”
The concerns came to the fore as Andy Burnham warned there is a “real risk” the party could split if he does not defeat Mr Corbyn.
The shadow health secretary said the party was at a “fork in the road” and could end up divided and irrelevant.
“Labour is at a fork in the road. I am now worried that, if we take the wrong turn, there’s a real risk that the party could split,” he told the Sunday Mirror.
“People who are being hit hard by this Tory Government urgently need Labour to come together and get its act together.”
Writing for the newspaper, ex-home secretary David Blunkett branded 66-year-old Mr Corbyn a representative of “the Old Left” who lacked answers to today’s problems.
“These spasms of self-indulgence are not new ... But now is the time for them to stop,” he insisted.
Mr Blunkett said he would be backing Mr Burnham for the leadership.