Labour has been accused of a “stitch-up” after scrapping its all-female shortlist in the race for the next mayor of Liverpool, with one candidate threatening legal action if the decision is not reversed. The party reopened the selection for a mayoral candidate on Tuesday shortly before ballots were due to go to party members, with none of the existing candidates invited to apply. Figures on the Labour left have accused the party of trying to keep Anna Rothery, the candidate endorsed by the former Labour leader, Jeremy Corbyn, off the ballot. Rothery said that she was “shocked by the party’s chaotic handling of the selection”. The Guardian understands she will seek an injunction if the decision is not reversed. Last week, the party “paused” the voting process in order to reinterview the candidates: Rothery, Wendy Simon and Ann O’Byrne. The three women, who are all Liverpool councillors, are well-known figures in the city’s political circles. Wendy Simon is currently interim mayor, Ann O’Byrne is a former deputy mayor and Rothery is the current lord mayor. The move seemed to have taken the candidates by surprise, with Rothery and O’Byrne rallying for votes on social media in the hours before the candidacy was reopened. The former chancellor, John McDonnell, tweeted that the “fiasco leaves the Labour bureaucracy wide open to charges of sheer incompetence or a political stitch-up or both. If there was a problem with any candidate it should have been dealt with earlier or is the problem the socialism of a possible winner?” Marcia Hutchinson, a committee member for the BAME Labour North West group, has organised a letter in solidarity with Rothery who, if elected, would have been the first black female mayor of Liverpool. More than 300 signatories state that it is “beyond transparent” that the purpose of this action is to stop the leading candidate being elected. “The decision to effectively invade the pitch to stop the match because their team is not winning is both morally and legally suspect,” the letter said. A Momentum spokesperson said reopening the selection process was deeply concerning. “Delaying ballots and then removing candidates with no explanation smells like a stitch-up to keep left candidates off the ballot.” In a statement, Rothery, who was backed by the union Unite, said: “Like many people across our city and our movement, I’m shocked by the party’s chaotic handling of the selection for Liverpool mayor. Instead of a positive, unifying campaign for our city, we are faced with what looks like an undemocratic failure of process.” She said that she stood as the city needed a “socialist mayor to clean up local politics, deliver a local Green New Deal and leave no one behind”. Rothery said that she welcomed more scrutiny in the process but not to “remove transparency and accountability”. “If the decision stands, then I will be left with no choice but to challenge it legally,” she added. One council insider said that Labour’s NEC had “taken its time waking up to the situation on the ground in Liverpool with both the police investigation and the government inspection”. In December, the government ordered an inspection into the council’s planning, highways, regeneration and property management functions after the arrest of Joe Anderson, the city’s mayor, and several council officers as part of an investigation into allegations of corruption in regeneration projects. Anderson denies wrongdoing but stepped back from his position and said he would not stand for re-election, prompting the selection race. Two of the three candidates – Ann O’Byrne and Wendy Simon – served as Anderson’s deputy mayor. O’Byrne previously held the regeneration brief but dramatically quit Anderson’s cabinet in 2018, accusing him of “not listening” to colleagues. A spokesperson for Labour said that after careful consideration, the party was reopening the selection. “We are committed to ensuring members are able to choose the right candidate to stand up against the Conservatives, lead Liverpool out of the coronavirus crisis and fight for the resources that the city desperately needs.” In an email to Liverpool Labour members, the party said ballots will be distributed from 8 March so members could choose from “an alternative shortlist of candidates”.