A long-awaited inquiry has found that bitter in-fighting between Labour moderates and Jeremy Corbyn supporters left the party “dysfunctional” and undermined its ability to hold the government to account.

The probe by Martin Forde QC said the party spent more time “occupied by factional differences than working collaboratively to demonstrate that the party is an effective opposition”.

His investigation was ordered by Keir Starmer after the leaking of an internal report into how the party handled accusations of anti-semitism, which laid bare the tensions that existed in the party under its former leader.

The leaked report, which ran to 860 pages, “quickly morphed into a wide-ranging critique of the factional attitude of senior professional party staff to the Jeremy Corbyn leadership”, Forde wrote.

It was pulled together when former general secretary Jennie Formby was in charge and was leaked in full shortly after Starmer became leader in 2020.

The leaked report contained hundreds of private WhatsApp messages between former officials, many of them derogatory, about Labour staff, members and pro-Corbyn MPs.

Among some of the accusations central to the report was that staff opposed to Corbyn deliberately sabotaged the 2017 general election campaign.

But Forde found that while there was an “increasingly bitter and factional rift in the party” there was no evidence to support claims that staff in Labour HQ wanted to the party to “do badly” in the election.

Forde’s report acknowledged that there was a “disagreement” over strategy between Corbyn’s office and the party’s campaigning headquarters but that he had not seen evidence of “bad faith”.

Those in the leader of the opposition’s office (Loto) wished to pursue a more “aggressive” strategy to win more seats, whereas those in the party’s HQ felt there should be a “defensive” strategy aimed at minimising losses and “shoring up” good MPs.

But Forde said: “We find that HQ staff genuinely considered that a primarily defensive strategy would secure the best result for the party, and we have not seen evidence to suggest such a strategy was advanced in bad faith.

“More broadly the evidence available to us did not support claims that HQ staff wanted the party to do badly in the 2017 general election.”

The claim that party staff scuppered efforts to win the 2017 poll was one of several made in the leaked report.

Among the more damaging claims were that party officials used a number of insults to describe senior Black MPs and officials including Diane Abbott, Dawn Butler and Clive Lewis.

And today Forde’s report concluded that MPs of colour and female MPs were “not always treated during the relevant period in the same way as their white/male counterparts — not just in terms of the abuse they received, but in terms of the level of instinctive respect they were afforded within the party and within parliament”.

“It is incumbent on party staff to recognise this failure and to continue to work to ensure that it does not persist,” Forde said.

On racism in the party more widely, Forde found that the “fundamental problem” was that “people who are committed to progressive politics find it difficult if not impossible to accept that they might have acted in a way which was discriminatory”.

He continued: “There seems to us to be a tendency among party staff to believe that they are insulated from the ills of their society — the same dynamic which was, in our view, behind the failure of the elected leadership to countenance that (as lifelong anti-racists) they could be behaving in a way which perpetuated anti-Semitism.

“The evidence clearly demonstrated that a vociferous faction in the party sees any issues regarding anti-Semitism as exaggerated by the right to embarrass the left.

“It was of course also true that some opponents of Jeremy Corbyn saw the issue of anti-Semitism as means of attacking him. Thus, rather than confront the paramount need to deal with the profoundly serious issue of anti-Semitism in the party, both factions treated it as a factional weapon.”

The Forde report found that the disciplinary process in the Labour Party was also “potentially prone to factional interference”.

“We found a disciplinary process not fit for purpose during the period we investigated and therefore one that was potentially prone to factional interference,” he said.

Moving forward, Forde also called for “constructive engagement” with the findings contained in the 138-page review.

“There is a culture of intellectual smugness which exists at the extremes of the political spectrum the party represents. In the past this has led to the dismissal of valid, albeit sometimes uncomfortable, views. It must now come to an end.”

A Labour Party spokesperson said: “The Forde report details a party that was out of control.

“Keir Starmer is now in control and has made real progress in ridding the party of the destructive factionalism and unacceptable culture that did so much damage previously and contributed to our defeat in 2019.”