Labour is set to accuse five former party staff, including Seumas Milne and Karie Murphy, of leaking a controversial internal report into antisemitism, prompting them to hire lawyers to defend themselves against the claims.

Labour has lodged papers at the high court seeking to place responsibility for the leak on Milne, Murphy, Georgie Robertson, Laura Murray and Harry Hayball, according to lawyers for the five. Carter Ruck said they would “vigorously defend themselves” in legal proceedings and seek full reimbursement for their costs.

The report, The work of the Labour Party’s Governance and Legal Unit in relation to antisemitism, 2014-2019, was leaked in April 2020 with damaging consequences.

The document, which was intended for, but never submitted to, an investigation by the equalities watchdog, alleged that staff had worked to undermine Jeremy Corbyn’s leadership and sent a number of abusive messages about senior figures. Critics of the report said it was leaked in an attempt to “smear whistleblowers” who had exposed antisemitism.

On becoming Labour leader, Keir Starmer launched an independent inquiry into the report and a second one by Martin Forde QC. Since then, the party has become mired in legal action, with Labour facing a claim against it by some of the staff named in the report.

Emilie Oldknow, one of those taking legal action against the party, unsuccessfully tried to force the party to disclose the names of those it believed were involved in the leak earlier this year.

It is not clear why Labour is now naming the five in legal documents, and the party on Monday declined to comment on ongoing legal action. Milne, a former Guardian journalist, worked as Jeremy Corbyn’s head of communications, while Murphy was his chief of staff.

However, Carter Ruck said in a press release that the party was trying to deflect blame for the leak on to the five former staff in a bid to avoid legal costs.

“To the extent that the Labour party has explained its proposed action, it is clear that it will be naming the individuals in an attempt to deflect on to them its own liability in claims brought by a group of claimants who are suing the party over the leak as well as the party bringing a related claim direct against the five,” the law firm said.

A spokesperson for the former Labour party employees said: “The individuals entirely reject these baseless claims. They did not leak the report. They fully cooperated with the party’s investigation by an independent external investigator, and with the inquiry led by Martin Forde QC.

“They understand that neither of those investigations concluded that they were responsible. The party has already acknowledged in court that it cannot be certain who leaked the report and that its ‘case’ against them is circumstantial.

“But it is now trying to make them foot the bill for legal action brought against it. The party should be focusing on the deeply troubling evidence contained with the leaked report, rather than trying to wrongly scapegoat and victimise former staff who documented it, and who have not been accused by either of the independent investigations.”

It comes after a senior party official revealed that Labour is now spending more than GBP2m on legal fees every year. Johanna Baxter, a member of the ruling national executive committee from the party’s pro-Starmer wing, said costs used to be just 10% of that figure, about GBP200,000 a year.