Leftwinger Sharon Graham is on course to pull off a surprise victory in the race to succeed Len McCluskey as the general secretary of the Unite trade union.

Ms Graham, 52, is expected to become the union’s first female leader beating frontrunner Steve Turner, who was backed by Mr McCluskey, and moderate candidate Gerard Coyne.

Votes are due to be counted on Wednesday, but from a sampling of ballots on Tuesday, Ms Graham has a commanding lead, a fact conceded by rival campaigns.

A source in Ms Graham’s campaign has told Sky News she is “confident the result will turn out to be for her”.

In a text message to supporters, Steve Turner said: “It appears that Sharon has got it! Count is tomorrow but from the sorting today that’s clear that’s the case and Coyne is last!”

A source on the Coyne campaign has also admitted Ms Graham’s victory is highly likely: “It was close at the start but her lead has now been consistent for two days. She has run an effective under-the-radar operation.”

Sharon Graham’s team believe Mr McCluskey’s endorsement ‘absolutely’ damaged Steve Turner’s campaign, as did the support of Howard Beckett, a controversial candidate who pulled out earlier in the contest.

Ms Graham had resisted pressure from others on the union’s left to withdraw from the contest and support Mr Turner. She called Mr McCluskey’s endorsement of him “desperate”.

Earlier in the campaign she hit out at what she claimed was Mr Turner’s sense of entitlement: “There is a man running who thinks it’s his turn to be the next general secretary,” she said.

Her campaign team believe Mr Turner and his supporters were complacent: “They downed tools a fortnight ago. They thought it was in the bag.”

Ms Graham’s allies believe her core message – “to stop the obsession with Labour and focus fully and squarely on the worker” – chimed with union members.

Unite has 1.2 million members and is the Labour Party’s biggest donor. Ms Graham has previously said there will be “no blank cheque” for Keir Starmer if she becomes general secretary, but “if Labour do what they’re supposed to do to defend workers they will have no problem with me”.

Sharon Graham is a veteran of Unite workplace disputes and high-stakes negotiations, including protecting tens of thousands of British Airways workers from the threat of “fire and rehire” last year.

After being subjected to online criticism for staying in the race, she said: “Like any bully in a workplace, I don’t take much notice of it… I’m a tough cookie.”

She has described the union as “quite male” and “boysey”, claimed she knows “what it’s like to have glass ceilings” and argued “there are some times where actually other people get promoted over you”.

The official election result is due to be announced on Thursday.

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