Labour MP John Mann urges the party to halt its leadership contest amid fears that hard-left groups are signing up to back Jeremy Corbyn.
Conservative Party supporters have also been encouraged to exploit the new membership rules to help Mr Corbyn take over the leadership of the party, a move they say will make Labour unelectable.
There is speculation that 140,000 more people could now be eligible to vote than before the general election – with an unknown number signing up just to back Mr Corbyn.
Mr Mann told the Sunday Times: “It should be halted. 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.”
Militant Tendency were a Trotskyist faction who attempted to gain influence over the Labour Party in the 1980s by pursuing a policy of “entryism” – getting members into key positions of power.
Rival leadership candidate Andy Burnham said calls for the contest to be stopped were not “all helpful” and he did not believe there was any “wide-scale” infiltration.
But he said there was a “real risk” Labour could split if he does not defeat Mr Corbyn.
The shadow health secretary told Sky News: “If John has evidence then he needs to send it to the Labour Party, but we’re in the middle of a debate that is actually now finally capturing attention and I think that’s a good thing because I think the party needs to have this debate and then decide its future direction.
“What we need more than anything now is unity coming out of this contest. People might hear this call as an attempt by the establishment to kind of skew the race one way or another, I think we need to keep it positive.”
Labour backbencher Simon Danczuk said: “I don’t think we can (pause the contest), to be honest. I think we are where we are. The party is in a terrible place.
“It is quite clear there has been some movement to try and get people who are Conservatives to sign up to be supporters. I think there has been an orchestrated effort to do that.”
Labour MP John Mann urges the party to halt its leadership contest amid fears that hard-left groups are signing up to back Jeremy Corbyn.ictory for Mr Corbyn was both a “nightmare” and a realistic prospect, he added.
Speaking to the BBC’s Andrew Marr show, Mr Corbyn dodged questions about whether he was a Marxist, saying: “Marx obviously analysed what was happening in a quite brilliant way. The philosophy around Marx is absolutely fascinating.
“Does it all apply now? Well obviously philosophy applies at all times. Do we then take that as a way in which we ensure that people have reasonable security in their lives through public ownership of the major monopolies? I think that is a fair point to look at. It is not unpopular with the public.”
Asked whether he was a Marxist, Mr Corbyn said: “That is a very interesting question actually. I haven’t thought about that for a long time. I haven’t really read as much of Marx as we should have done. I have read quite a bit but not that much.
“I think Marx’s transition of history and the analysis of how you go from feudalism to capitalism and move on to a different stage is fascinating.”
He added: “I am not talking about the Labour Party being a revolutionary party. It certainly isn’t.
“What we are doing here is putting forward a view that the Labour Party has to offer a credible alternative that is true to the roots of the Labour Party, the roots of the Labour Party are essentially democratic, essentially socialist, essentially community.”
Mr Corbyn reiterated his desire to renationalise the railways and utility companies, saying Labour lost the last election because it was “too close to big business”.
He said: “The role of the state or government, of the community, is surely to provide people with some security in their lives, to provide them with a decent health service free at the point of use, to provide them with a welfare state that stops people falling into destitution. We are not doing that – we are making it worse.
“Where there are natural monopolies like Royal Mail, like the railways it seems to me counter-intuitive that we spend a great deal of money on investing in railway infrastructure, billions, and then we hand it over to train operating companies to run it.
“I think it is much better if we bring the rail system as a whole into public ownership. It’s not some extreme position.”