The race to succeed Len McCluskey as the leader of Unite the union is expected to become increasingly tense after the mainstream candidate Gerard Coyne disclosed to the Guardian that he has won enough nominations to reach the final ballot. The announcement is likely to prompt furious negotiations between Coyne’s leftwing rivals as they seek to persuade each other to stand down to keep him out. Sharon Graham, Steve Turner and Howard Beckett are expected to begin negotiations after the close of nominations on Wednesday as supporters seek to maintain control of the union’s industrial muscle before names are put forward for a final ballot of the union’s 1.2 million members. A close associate of Graham insists there is “not a cat in hell’s chance” she will make way for her two male rivals from the left and she plans to be the union’s first female leader. Supporters of the left candidates had grown increasingly concerned that Coyne, who has been a Labour member for more than 30 years and was narrowly defeated by McCluskey in a 2017 contest, would reach the threshold of 174 branch nominations that would allow him to make the runoff. The contest will be watched closely by Keir Starmer and the parliamentary Labour party. Unite has been a strong critic of his style and politics, and has maintained close alignment with Jeremy Corbyn and his former supporters. Coyne, a former regional organiser in the West Midlands and generally supportive of Starmer, has called for the union to focus on its members rather than controlling Labour, and has called for an independent probe into the use of members’ money to build a £98m conference centre in Birmingham. He said on Tuesday that he has passed the threshold. “I’m delighted to have received so much support from Unite members from across the union and can confirm I will be accepting the nomination. “Any suggestion that there will be a grubby backroom deal between the three continuity candidates would be both anti-democratic and a betrayal of Unite members right to choose … It will also show they are running scared and how desperate they are to block real change,” he said. Sources from within the candidates’ camps said that attempts have been made by senior figures in the party to reach out to Graham to persuade her to step down. A close associate of Graham, who is seeking to disentangle the union from Labour’s internal politics, said: “There is not a cat in hell of Sharon standing down. She has [nominations from] Hinkley Point, health trusts and in Edinburgh city council and a real breadth of support among Unite’s industrial branches.” “Given her profile and the level of support she has across the branches, maybe one of the men would like to step aside for a woman?” Left activists are concerned that three candidates would split the vote in a first-past-the-post leadership election, allowing Coyne to win. Beckett, the union’s former legal adviser who was recently suspended from Labour for saying that Priti Patel should be deported instead of refugees, has been a close aide to McCluskey. He is advocating an increase in the union’s strike fund from £40m to £50m. Turner, who has also been close to McCluskey, was seen as the favourite to win the contest after securing the support of the United Left faction. He has argued for more pragmatism from the union’s leaders. Both Beckett and Turner are expected to win more nominations than Graham, whose supporters say she is hoping for between 300 and 350. And both have argued for the left not to split the vote. There are about 3,000 Unite branches, and each candidate needs to secure 174 to guarantee a place on the final ballot to be announced on Thursday, with a final result announced in late August. Turnouts tend to be low – last time just 12% of the union’s members voted – which can result in surprising results. Backroom deals between candidates often take place to help factions to maintain control. Unite has been Labour’s biggest donor and was criticised for having too much influence over the party under Corbyn. The union maintains influence over the party’s ruling National Executive Committee. Beckett has said he would consider withholding funds from Labour. The election comes after Unison, the UK’s biggest union, and GMB, another major Labour funder, both chose new general secretaries who are broadly supportive of Starmer.