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The leader of one of the UK's biggest trade unions has had his vote in the Labour leadership election rejected.

Mark Serwotka, general secretary of the Public and Commercial Services (PCS) union, voted for Jeremy Corbyn, but was told his vote would not be counted.

Mr Serwotka has previously publicly criticised Labour's "move rightwards".

Labour said it would not comment on individual cases but said people "who don't share the aims or values of the Labour Party don't get a vote".

Meanwhile, Labour has decided it will now allow data collected by campaigners knocking on doors during the general election to check the views of voters, the BBC understands.

'#LabourPurge'

The PCS, which represents civil servants, is not affiliated to the Labour Party.

Mr Serwotka's wife Ruth tweeted about the vote, saying: "Mark been prevented from voting in a Labour Movement election as an affiliated member. I'm very interested to hear the reasons. #LabourPurge."

Image caption Labour MP Ian Austin says "tens of thousands" of voters could be members of other parties

In an interview with the New Statesman in 2011, he said he had voted for the Green Party in the 2010 election and added: "Growing up in Wales, it was Labour, Labour, Labour. But [since] its move rightwards and embrace of the markets, Labour doesn't speak for me."

Mr Serwotka is among 3,200 people who have so far been prevented from voting in the leadership election after signing up as affiliated members, with many more expected to follow.

Labour MP Ian Austin said "tens of thousands" of people who are members of other parties could be able to vote.

Mr Austin - who was an aide to former Prime Minister Gordon Brown - said Labour officials had randomly sampled registered supporters, who had signed up to vote in the contest for £3, and that 4.4% had admitted they "actively supported" other parties.

'Not thought through'

"With hundreds of thousands of people joining, if 4.4% support other political parties, that could mean tens of thousands of our opponents are able to take part in this process.

"I don't think it's a very good advertisement for the Labour Party," he added.

Image caption Yvette Cooper, Jeremy Corbyn, Liz Kendall and Andy Burnham are battling to be Labour's leader

The new system, which among other things made union members who want a vote in leadership contests sign up as affiliated supporters, was introduced last year by previous Labour leader Ed Miliband.

Mr Austin suggested it had been a knee-jerk reaction to a candidate-selection scandal in Falkirk and had not been "thought through".

It was voted through by an overwhelming majority of party members at a special Labour conference.

But Labour officials have been under pressure from several leadership candidates to tighten up the vetting process for new supporters.

Labour is now understood to have allowed canvas data to be used to vet voters, despite an internal committee previously rejecting such demands, the BBC's political correspondent Ross Hawkins said.

The party's general secretary Iain McNicol is thought to have written to candidates about the change of heart earlier on Wednesday.

It has also been agreed that the published results of the leadership election will be broken down into full members, affiliated supporters and registered supporters.