Labour MPs have been ordered to vet people who have applied to join the party, amid growing concern that Trotskyists and others are signing up to vote for Jeremy Corbyn in the leadership election.

Harriet Harman, Labour’s acting leader, has sent all the party’s MPs the names of the recruits in their constituencies, and asked them to weed out known opponents.

Around 65,000 of Labour’s 275,000 full members have joined since the May general election. Another 35,000 have paid £3 to become registered supporters, which gives them a vote in the leadership contest. A further 35,000 trade union members have signed up for free as affiliated supporters. The cut-off date is 12 August, after which voting will start.

Ms Harman’s letter, seen by The Independent, says: “As an MP, your local knowledge and information is important to uphold the integrity of the leadership election.”

Labour leadership: The Contenders Show all 4 1 /4 Labour leadership: The Contenders Labour leadership: The Contenders Jeremy Corbyn Jeremy Corbyn started off as the rank outsider in the race to replace Ed Miliband and admitted he was only standing to ensure the left of the party was given a voice in the contest. But the Islington North MP, who first entered Parliament in 1983, is now the firm favourite to be elected Labour leader on September 12 after a surge in left-wing supporters signing up for a vote. PA Labour leadership: The Contenders Liz Kendall Liz Kendall has been labelled the Blairite candidate throughout the contest, which partly explains why she has failed to attract the support needed in a party that has drifted even further from the centre-ground of British politics since the election. She has faced criticism over her relative lack of experience, having only served as an MP since 2010 and having no experience of ministerial or shadow cabinet roles. But that very lack of experience allowed her to initially make a pitch as the only candidate offering real change and a real break from the Blair/Brown/Miliband years, until Jeremy Corbyn entered the race and shifted the whole debate to the left. She is set to finish a disappointing fourth. PA Labour leadership: The Contenders Andy Burnham Andy Burnham started out as the front-runner in the leadership election, seen as the candidate of the left until Jeremy Corbyn entered the race. The former Cabinet minister has found himself squeezed between the growing populism of Corbyn’s radical agenda and the moderate, centre-left Yvette Cooper, not knowing which way to turn. It has attracted damaging labels such as ‘flip-flop Andy’, most notably over his response to the Government’s Welfare Bill. He remains hopeful he can win enough second preference votes to take him over the 50 per cent threshold ahead of Corbyn. PA Labour leadership: The Contenders Yvette Cooper.jpg Yvette Cooper has put her experience and achievements in government at the heart of her offer to the Labour party. She played a key part in setting up Sure Start in Tony Blair’s government and has pledged to continue her record on delivering for young families by promising a “revolution in the way families are supported” by introducing universal free childcare. She has also championed her role as a full-time working mother, taking pride in telling audiences that she does the school run for the kids before her day starts as a politician. But she has been criticised for being too wooden and lacking in passion and her attacks on Liz Kendall for “swallowing the Tory manifesto” at the start of the leadership contest have been criticised for helping Jeremy Corbyn brand all three mainstream candidates as ‘Tory-lite’. PA

She adds that the names of new recruits are being sent to local party officials “in real time”. They have been asked to “check there aren’t any people known to them to be members of other parties, or who do not support the Labour Party”.

Ms Harman’s intervention was welcomed last night by supporters of the other leadership candidates – Andy Burnham, Yvette Cooper and Liz Kendall. One MP said: “There is real concern in all three camps. There is real evidence that the non-Labour hard left is joining to make sure Jeremy wins. That cannot be good for the health of the party.”

Mr Corbyn has played down claims about the return of “entryism”, used by the Militant Tendency in the 1980s, and insisted that only people who support Labour should sign up to vote in the contest.

Alan Johnson, the former Cabinet minister, urged Labour members to “end the madness” of their flirtation with Mr Corbyn and elect Ms Cooper. The endorsement is a significant boost for her because Mr Johnson is widely respected in the party.

He said: “I have never had the ambition or the appetite that this job requires. Neither has Jeremy Corbyn – as he’s honest enough to admit.”

Former Labour Cabinet minister, Alan Johnson (Teri Pengilley)

He added that Ms Cooper has “the intellect, the experience, to succeed in this most difficult of roles”.

Mr Johnson’s intervention is part of a drive by senior Labour figures to halt Mr Corbyn, who dismissed the personal criticism: “It is the name-calling ... that drives people away. Our campaign is not getting involved. We aren’t doing rebuttals. We’re not interested.”