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One of Keir Starmer’s most vociferous critics has failed in his bid to win the Left’s endorsement as the successor to Len McCluskey as the head of the Unite trade union.

Howard Beckett, who had campaigned explicitly on a platform of attacks on the Labour leader, was confirmed as having lost to Steve Turner for the crucial endorsement of the United Left faction of the 1.2 million member-strong union.

The result will have a key impact on the Labour party, where Unite’s presence on the ruling National Executive Committee (NEC) as well as its role as the biggest donor has been highly influential.

Turner, Unite’s assistant general secretary with responsibility for manufacturing, won the United Left election by 370 votes to 367 after its hustings on Saturday but the result was swiftly called into question by Beckett’s supporters who claimed irregularities in the ballot.

After receiving two reports on the conduct of the election, including one from the independent scrutineers running the ballot, United Left’s ruling committee has now “overwhelmingly” endorsed Turner’s victory.

HuffPost UK understands that the committee dismissed claims that one dead member of the union had been allowed to vote, and confirmed that all those who had been eligible to vote had been allowed to do so.

Former Labour general secretary Jennie Formby – who had supported Beckett – was barred from voting as her subscription to the union was not up to date, sources have said.

Some 230 of Beckett’s 367 votes came from Scotland, where the union had launched a big recruitment drive at the Grangemouth oil refinery run by INEOS, whereas Turner’s were spread over the whole of the UK and Ireland.

Candidates for the Unite Left nomination usually agree before the contest not to run against each other in the wider election for general secretary and Beckett’s hopes of succeeding McCluskey appear dashed as a result.

Beckett had tweeted the day before the hustings that “regardless of the outcome the left will emerge from this united”. The tweet appeared to have been deleted following the hustings and ballot result.

If Beckett were to run in his own right, he risks splitting the Left vote and allowing centre-right candidate Gerard Coyne to win.

In a statement late on Wednesday night, United Left chair Jim Kelly said the election had been “a profoundly democratic process” with the largest turnout and selection vote in its history. “Steve Turner has now been confirmed as the United Left nominee for the position of Unite General Secretary,” he said.

A key ally of former Jeremy Corbyn chief of staff Karie Murphy, Beckett has been vociferous in his criticism of Starmer in his role as one of Unite’s representatives on Labour’s NEC.

He has attacked the Labour leader for pushing for the reopening of schools and for his sackings of Rebecca Long-Bailey, Ian Lavery and Richard Burgon.

In one tweet he warned Starmer and Boris Johnson against trying to “dump the pandemic fallout on the working class”.

Beckett’s campaign included a pledge, much ridiculed by his opponents, to spend union members’ money on a “Unite TV” station that would be a left-wing rival to Netflix and the BBC.

Centre-right candidate Gerard Coyne is expected to make another bid to lead Unite in the general secretary election, which will start at some point next year, although Turner will be seen as the favourite given his close links to McCluskey.

McCluskey’s term ends in April 2022 but the election to succeed him will take at least 6 months and union sources have confirmed that the race will have to start in 2021.

Turner, who has been a lifelong member of the union and shop steward, is firmly of the Left and had backed Long-Bailey for leader. But he is seen as more of a pragmatist focused on workers’ rights than publicly challenging the party leadership.

He recently told the New Statesman: “I’m a democratic centralist. It’s our job to steer and guide and influence the leadership, not to have a public spat with the leader of the Labour Party.”

Turner added that “I do not support … a war of attrition inside our party which is being waged by some other candidates”, itself seen as a reference to rival Beckett.