Labour leader Sir Keir Starmer has launched an inquiry into the leak of an internal report into the party's handing of anti-Semitism claims.
The 860-page dossier says anti-Jeremy Corbyn sentiment among Labour staff hindered efforts to tackle the issue.
Sir Keir said the inquiry will look at why it was commissioned, its contents, and how it came into the public domain.
The Campaign Against Anti-Semitism said the report was leaked as an attempt to "smear whistleblowers".
It is understood the document - dated March 2020 - is a draft drawn up to help inform the party's responses to an investigation by the Equality and Human Rights Commission (EHRC).
The watchdog launched a probe into anti-Semitism within Labour in May 2019 after a complaint from the Campaign Against Anti-Semitism.
A Labour spokesman said the party had submitted "extensive information to the EHRC and responded to questions and requests for further information" - but none of that detail was included in the leaked report.
In a joint statement with deputy leader Angela Rayner, newly elected leader Sir Keir said: "We have seen a copy of an apparently internal report about the work of the Labour Party's Governance and Legal Unit in relation to anti-Semitism.
"The content and the release of the report into the public domain raise a number of matters of serious concern.
"We will therefore commission an urgent independent investigation into this matter."
He said they have asked for "immediate sight" of any legal advice the Labour Party had received about the report.
"In the meantime, we ask everyone concerned to refrain from drawing conclusions before the investigation is complete and we will be asking the general secretary to put measures in place to protect the welfare of party members and party staff who are concerned or affected by this report," Sir Keir said.
By BBC Political Correspondent Helen Catt
For Jeremy Corbyn's enduring supporters, this report will back up what they've thought all along: that there were those in the party establishment who never accepted their man as leader.
The dismissive references to "Trots" and "mentalists" in WhatsApp messages, apparently between staff working for the party at the time, will clearly cause anger.
However, it's the allegation that Labour staff worked against a win for Mr Corbyn in the 2017 election that is likely to be most incendiary, if proven.
Explosive as all this may be, some have questioned how much bearing it has on what the report says it's about: the handling of anti-Semitism disciplinary cases.
So, Sir Keir Starmer has promised an investigation, not just into the content, but into how the document came into being and got into the public domain (and, with it, unredacted information about some members).
It seems like a signal that he won't favour any one faction but it also suggests that his stated aim of uniting the party will be far from easy.
For Sir Keir, the timing of this latest very public row could not be worse, as he tries to focus on his party's response to the coronavirus epidemic.
Shadow chancellor Anneliese Dodds said she "wasn't aware of this report until it was leaked" and that the party needed to understand the circumstances under which it was leaked, because it contained sensitive information.
"We also need to look at the cultures that appear to have been described in that report and that needs a proper independent review, that is going to be delivered, and I think that will answer any questions that people have," she said.
The leaked document said Labour's latest investigation into anti-Semitism in the party revealed "a litany of mistakes, deficiencies, and missed opportunities to reform, develop and adapt a clearly failing disciplinary system", and "disproved any suggestion that anti-Semitism is not a problem in the party".
It also said the "rigorous and far-reaching reforms necessary to bring the party's procedures up to standard were not undertaken early enough".
However, the document claimed the "extremely strained relationship" between Mr Corbyn and Labour headquarters during his tenure had stopped oversight "over the disciplinary process", with the party's management being "generally more obstructive than it was constructive".
It included transcripts of WhatsApp messages between party staff, which it said showed opposition to Mr Corbyn.
At the extreme, some staff members seemed to have "taken a view that the worse things got for Labour, the happier they would be since this might expedite Jeremy Corbyn's departure from office," the report said.
It claimed to have found "no evidence" of anti-Semitism complaints being handled differently to other forms of complaint, and said that in 2019, half of all anti-Semitism complaints came from a "one individual" who is accused of being "rude and abusive" to party staff.
The report also claimed there had been a "steady, if imperfect, rate of improvement" after Mr Corbyn's ally, Jennie Formby, took over as general secretary from the former post holder, Ian McNicol.
The document praised measures taken by Mr Corbyn since 2018, including the introduction of fast track expulsions, describing the moves as "transformational".
It added: "These safeguards ensure that the past mistakes in the handling of anti-Semitism complaints cannot be repeated now."
But the Campaign Against Anti-Semitism, which reported Labour to the EHRC, said the report "serves as an exhibit of the party's failure to address the crisis" and should be handed over to the watchdog.
The organisation's chief executive, Gideon Falter, said: "In the dying days of Jeremy Corbyn's leadership, the Labour Party appears to have invested in a desperate last-ditch attempt to deflect and discredit allegations of anti-Semitism.
"Rather than properly dealing with cases of anti-Semitism and the culture of anti-Jewish racism that prevailed during Mr Corbyn's tenure, the party has instead busied itself trawling through 10,000 of its own officials' e-mails and WhatsApp messages in an attempt to imagine a vast anti-Corbyn conspiracy and to continue its effort to smear whistleblowers."
Labour has been plagued with allegations of anti-Semitism since 2016.
Mr Corbyn held an internal investigation early on in his tenure, but it was widely criticised by Jewish members of the party, with a number - including MPs - leaving over his handling of the row.
The party's new leader, Sir Keir Starmer, has apologised to the Jewish community for the ongoing issue.
He has been praised by leaders for "achieving more in four days" than Mr Corbyn did "in four years" on tackling anti-Semitism.