One of the UK’s most high-profile freedom of expression lawyers has accused Labour of “spin and dissembling” for denying it attempted to stop two female ex-staffers from speaking about sexual harassment.

In a highly unusual intervention, Mark Stephens, who represented Georgie Robertson and Laura Murray, said there were “high public interests at play” and said the party was “undermining their credibility” by issuing statements denying it had tried to offer them non-disclosure agreements.

Stephens, a trustee of Index on Censorship and who sits on the FCO Free Expression advisory board, has previously worked on the McLibel case and represented Julian Assange and Salman Rushdie, leading the department at Howard Kennedy LLP.

His letter was also backed by one of the country’s leading experts in labour law, Lord Hendy QC, a Labour peer, who told the Guardian that he “can’t see how it would be possible to deny that [the settlement the women were offered] is anything other than ‘a non-disclosure agreement’”.

Both women worked under the previous Labour leader, Jeremy Corbyn, and made separate complaints of sexual harassment against the same individual. One of them raised her experience in January 2020, and they both revealed their experiences in March 2020.

Robertson, a former press officer, and Murray, who was head of complaints, resigned from the party but refused to sign legal agreements with a confidentiality clause and left without payouts.

The former official accused of harassment was temporarily suspended but denied the allegations. Labour issued a statement to the Mail on Sunday saying no non-disclosure agreements (NDAs) “have been proposed to any member of staff alleging sexual harassment since [Keir] Starmer took over as leader”.

But Murray and Robertson cite documents showing the party’s most senior lawyer setting out confidentiality clauses to protect the party and the individual they accused.

In his letter to the Guardian, Stephens called both women “brave” and said that the clauses proposed by the party “would have prevented them taking legal action against their alleged harasser”.

He wrote: “It is unusual for a lawyer to write a letter of this kind to a newspaper, but there are high public interests in play when a political party makes an official statement denying that it proposed NDAs to cover up allegations of sexual harassment, and now adds insult to injury by claiming that it takes ‘any complaints of sexual harassment extremely seriously’ and that these are ‘fully investigated.’”

He said that the statements issued by Labour in media reports – including to the Guardian – should be corrected or it would be tantamount to the party “covering up the appalling things that they believe happened to them”.

Hendy, who has a long record as a barrister for trade unions, told the Guardian he agreed with Stephens, having reviewed the documents. “Confidential information is defined to include information relating to the business, affairs and ‘details’ of, amongst others, employees,” he said.

“The definition also includes information relating to ‘your work’. Someone who has been sexually harassed at work by a fellow member of staff is not therefore permitted to disclose (or retain) any information about it. I can’t see how it would be possible to deny that is anything other than ‘a non-disclosure agreement’. In fact, the offer is yet more restrictive since it forbids any claim being made ‘from or relating to’ ‘your grievance’.”

Labour received a long-awaited independent report by Martin Forde QC which looked at internal party culture and factionalism, which said the party had to make further progress against discrimination.

“It’s clear that the party cannot address this unacceptable culture while it continues to promote terminological inexactitudes about its handling of allegations of mistreatment of female employees, particularly where NDAs are contrary to Labour policy,” Stephens wrote.

The case of the two women have been raised by members of the party’s national executive committee and by female MPs in the party – including at meetings this week with the party’s general secretary, David Evans.

Stephens told the Guardian he would be willing to share all the documents concerning the case with members of parliament and anyone else appropriate.

“Such matters are not susceptible to spin and dissembling – they are far too serious for that,” he said. “The truth must be set free.”

The Guardian understands that NEC member Gemma Bolton attempted to raise the case at the meeting on Tuesday but was told media reports were inaccurate.

Both Robertson and Murray are two of five ex-staff members accused by Labour of leaking an inflammatory report containing unredacted private WhatsApps, many of them abusive about Corbyn and his allies. All deny leaking the report. In his inquiry, which was commissioned in the wake of the leaking of the report in 2020, Forde said he would not form a view on who leaked it.

Labour said it had no further comment beyond its previous statements.