THE safety of the ballot to select the next UK Labour leader has been questioned after nearly 75,000 newly registered party “supporters” signed up for a vote in the contest.

Party sources believe thousands of the new recruits may be invalid as they are believed to include non-Labour backers who are trying to install left-winger Jeremy Corbyn as leader.

Labour has already nullified a number of applications and is in the process of validating 30,000 more.

The race to succeed Ed Miliband is a four-way contest between MPs Liz Kendall, Andy Burnham, Yvette Cooper and Corbyn.

An opinion poll last week suggested that the left-wing candidate Corbyn was on course for victory.

He is also expected to get over 100 nominations from constituency Labour parties.

Supporters of the other candidates claim the contest is being skewed by the leadership rules bequeathed by Miliband.

Following the Falkirk selection debacle, Miliband replaced the old electoral college with a form of one-person-one-vote that gives every party member a single vote.

However, the franchise also includes two new tiers of “supporter”: members of affiliated trade unions who can register for free; and any member of the public who pays £3.

The latest figures show that 17,830 have become registered supporters through the £3 route, while 25,338 individuals will get a vote as affiliated supporters.

Another 30,000 affiliate applications are currently being processed which, if validated, would mean 73,168 new voters in the contest.

This is the equivalent of nearly 30% of the 253,566 fully-paid up Labour members.

In last week’s You Gov poll - which included registered supporters - Corbyn had a seventeen-point lead over Burnham and the snapshot forecast a victory for the left-winger.

Party sources believe the new system is easily manipulated by those on both the anti-capitalist Left and Tory Right, who are united by a hostility to Labour and desire to help Corbyn for their own particular ends.

Tories believe a Corbyn leadership would damage the party. Those on the left feel Corbyn represents the 'soul' of the party.

Some Conservative backers have been open about wanting to register as supporters.

New recruits simply have to tick a box which states they support the “aims and values” of Labour, a commitment that is impossible to prove unless the supporter is an identifiable backer of another party.

One source said: “After the Falkirk debacle, members of the Labour party would have been hoping that the system to elect a new leader would be robust. It appears that this is not the case and people are very concerned about whether the result will be viable. The system doesn’t look safe.”

Another fear expressed is that, given supporters can register as late as August 12, some trade unions have been using the past few weeks as a recruitment drive to give as many of their members as possible a vote.

Another insider said: “There should have been a rule saying that only supporters who had been registered for 6 months could vote. This would be a sign of commitment to the party, rather than a free-for-all in which non Labour supporters are signing up for the sole purpose of influencing our contest. After the leader is chosen, we won’t hear from these people ever again.”

Meanwhile, Cooper has hit out at alleged sexism from Burnham supporters in what is becoming an increasingly bitter and divisive contest.

Cooper said: "We can't go back to an old fashioned Labour party - not just back to the politics of the 80s but of the politics of the 50s - treating women as incapable of the top jobs, and a party led by two men...Liz [Kendall] has been asked about her weight, I've been asked on [BBC Radio 4's] Woman's Hour of all places about whether I can possibly do this job because of my husband, and any talk about me being a working mum has been used as a sexist way to divide Liz and I, and criticise Liz for not having children."

A row broke out over a Times article written by shadow justice secretary Lord Falconer, headlined "Women are not tough enough to lead Labour".

In the article, the Burnham supporter said "neither Yvette or Liz were able to steer the party through the challenging years ahead".

However, Lord Falconer insisted he was horrified by the headline and had never implied their gender was an issue.

A Labour spokesperson said: “While we want the widest number of supporters to have their say in Labour’s leadership contest, the Labour Party has a robust system to prevent fraudulent or malicious applications. All applications are verified against the Electoral Register, and any who are identified by our verification team or by local Labour branches as not sharing the aims or values of the Labour Party will be denied a vote.”