Labour deputy leader Tom Watson has demanded that the party publishes its submission to an inquiry into anti-Semitism allegations.
He said he had a right to see documents handed to the Equality and Human Rights Commission (EHRC).
Writing to Labour’s general secretary, he also criticised the party’s response to claims on the BBC’s Panorama about its handling of anti-Semitism cases.
Labour has said claims in the programme were inaccurate.
Speaking to Panorama, which aired on Wednesday, ex-party officials claimed senior Labour figures had interfered in the disciplinary process of dealing with accusations of anti-Semitism.
Labour has insisted the claims were inaccurate and made by “disaffected” former staff.
In a letter to Labour General Secretary Jenny Formby, Mr Watson said the party’s response had failed those who spoke to the programme and “breached all common standards of decency”.
“The way that they have been smeared, including by Labour spokespeople, is deplorable,” he wrote.
“Even if some in the party did not want to hear what they had to say, it is unacceptable to attempt to undermine their integrity and characters in this manner.”
He said Ms Formby had “insisted” members of the shadow cabinet should not have the right to see the EHRC submission, but added: “I disagree.”
“Only sunlight can disinfect Labour of anti-Semitism now,” he wrote.
But shadow home secretary Diane Abbott, a close ally of Labour leader Jeremy Corbyn, tweeted that Mr Watson knew “perfectly well that he cannot make ‘demands’ of Jennie Formby”.
She added that he was “very wrong to imply that she is dealing with this matter with anything less than her usual professionalism”.
A Labour source said Ms Formby had offered to meet Mr Watson to provide him with the party’s response to the EHRC probe.
The EHRC launched a formal investigation in May into whether Labour had “unlawfully discriminated against, harassed or victimised people because they are Jewish”.
The Jewish Labour Movement said on Thursday that more than 30 whistleblowers, including current Labour staff, would submit evidence to the inquiry.
Labour’s disputes team is supposed to operate independently from the party’s political structures, including the leader’s office.
BBC Panorama spoke to former party officials, who alleged they had to deal with a huge increase in anti-Semitism complaints since Mr Corbyn became Labour leader in 2015.
Some of the staff spoke to the programme despite having signed non-disclosure agreements (NDAs) when they left.
Eight former officials who worked in the team and dealt with anti-Semitism cases claimed to the BBC that:
- The leader’s office was “angry and obstructive” when it came to the issue
- Officials brought in by the party’s general secretary, Jennie Formby, “overruled” some of their disciplinary decisions and “downgraded” punishments to a “slap on the wrist”
- Seumas Milne, one of Mr Corbyn’s closest aides, laughed when advised by a long-serving party official about what Mr Corbyn should do to tackle anti-Semitism in the party
- On one occasion, Mr Corbyn’s office ordered batches of anti-Semitism complaints to be brought to his Commons office for processing by his aides
Labour has rejected claims of interference and described the programme as “seriously inaccurate” and “politically one-sided”.
Shadow chancellor John McDonnell – a close ally of Mr Corbyn – said current staff had put in complaints to the BBC about the accusations made in the Panorama programme.
“I have always said from the very beginning [the process of dealing with complaints] was too slow and not ruthless enough, but it has improved dramatically now,” he said.
“I think it is important that we listen to what has been said and look ourselves at what is happening, but what we’ve got now is two groups of staff challenging the accuracy of [the accusations] so we will have to look at that.”
But Labour’s shadow women and equalities minister Dawn Butler – who is a supporter of Mr Corbyn – said the party “must acknowledge the deep hurt caused to our Jewish brothers and sisters”.
Labour has been engulfed in a long-running dispute over anti-Semitism within its ranks, which has led nine MPs and three peers to leave the party.
The leadership has been accused of failing to get to grips with the problem, with allegations of hundreds of complaints against members remaining unresolved.
But Labour said it “completely” rejected any claims it was anti-Semitic.
It accused the Panorama programme of being a “seriously inaccurate, politically one-sided polemic, which breached basic journalistic standards, invented quotes and edited emails to change their meaning”.
The party said that “no proper and serious attempt was made to understand our current procedures for dealing with anti-Semitism, which is clearly essential to reach a fair and balanced judgement”.
It added: “Since Jennie Formby became general secretary the rate at which anti-Semitism cases have been dealt with has increased more than fourfold.
“We will build on the improvements to our procedures made under Jennie Formby, and continue to act against this repugnant form of racism.”