Labour’s former director of governance under Jeremy Corbyn issued a warning that an internal party  report into antisemitism that was leaked onto the internet in April was “misleading” and “without true context”, a leaked document has shown.

Thomas Gardiner, the party’s most senior lawyer under the previous leader,  also said that the document relied on confidential private WhatsApp and other messages which he said were “a clear and unacceptable breach of confidence.”

Labour is now facing up to 50 legal complaints over the leaking of the report – many from Jewish party members who had details of their complaints about antisemitism logged.

The Guardian reported on Friday how Mr Gardiner’s comments, sent in an email to the party’s then general secretary, Jennie Formby, will be a key piece of evidence for the complainants, who include the former general secretary Lord Iain McNicol.

They are suing the party for data breaches and libel within the leaked 851 page report.

One of the 32 claimants told the newspaper the evidence in Mr Gardiner’s email was “hugely damaging” for Labour.

“Those looking to take legal action against [the] party for the leaked report have long argued that the messages have been edited to create a misleading and damaging impression. What this email suggests is that this is exactly what Labour knew it was doing,” the claimant said.

Supporters of Mr Corbyn, including senior figures in the powerful Unite trade union, have attempted to use the report to claim that the former leader was hampered in his bid to tackle antisemitism in the party.

According to documents seen by the Guardian, Mr Gardiner wrote the email to Ms Formby to register his concerns on April 11.

He said that its authors were trying to “lay false blame” on him for failings in the complaints process.

He continued: “I also object further on the grounds that large sections of the report have been compiled using email searches which were not authorised for the authors to undertake including improper searches of my email account.

“The report also includes confidential private WhatsApp and other messages – a clear and unacceptable breach of confidence. Further, these messages are presented selectively and without their true context in order to give a misleading picture.

“I realise it will not be helpful not to further use the report, given that it has apparently been leaked in a previous version, but I must register this formal objection.”

The leaked report – which is subject to an independent inquiry led by Martin Forde QC - was commissioned by Ms Formby.

It contained leaked WhatsApp messages referring to Corbyn-supporting party staff as “trots” and conversations referring to the former director of communications Seumas Milne as “dracula”.

There were also mentions of Corbyn’s former chief of staff, Karie Murphy, as “medusa”, a “crazy woman” and a “bitch face cow” that would “make a good dartboard”.

Ms Formby, who stood down as general secretary in May, declined to comment on Mr Gardiner’s email.

“In view of the ongoing independent inquiry it would not be appropriate,” she said.

A Labour party spokesperson said: “It would not be appropriate to comment further while investigations are being carried out.”

Mr Gardiner, who stood down as head of the governance and legal unit in June, was also approached for a comment.