Sir Keir Starmer has defended Labour's decision to suspend Jeremy Corbyn from the party after the former leader's reaction to a report on anti-Semitism.
The UK's human rights watchdog says Labour broke the law by failing to stamp out anti-Jewish racism in its ranks when Mr Corbyn was leader.
Mr Corbyn said the scale of Labour anti-Semitism had been "dramatically overstated" by his opponents.
This comment prompted Labour to suspend Mr Corbyn from the party.
The unprecedented move was defended by Sir Keir - who took over as leader from Mr Corbyn six months ago - as the "right" thing to do.
He also said there was no reason for the party to descend into "civil war" over it.
But former shadow chancellor and close ally of Mr Corbyn, John McDonnell, said it seemed the party was "drifting towards a hell of a row over use of language, misinterpretation, followed by overreaction".
Sir Keir told BBC Radio 4's Today programme: "I made it clear the Labour Party I lead will not tolerate anti-Semitism, neither will it tolerate the argument that denies or minimises anti-Semitism in the Labour Party on the basis that it's exaggerated or a factional row."
Mr Corbyn's allies on the left of the Labour Party have reacted with anger and dismay to his suspension, and have vowed to help get him reinstated.
General secretary of the Unite union, Len McCluskey, called it "an act of grave injustice which, if not reversed, will create chaos within the party and in doing so compromise Labour's chances of a general election victory".
He called on Sir Keir "to work across the party on a fitting and unifying way forward" so that the Equality and Human Rights Commission's recommendations on stamping out anti-Semitism can be implemented.
Mr McCluskey urged members "angered by this suspension not to leave the party but to support moves to find a better way through".
Others, including Labour MP Margaret Hodge, Parliamentary chair of the Jewish Labour Movement, said it was the right decision.
Ms Hodge said he party was "finally saying enough is enough, anti-Semitism can never be tolerated".
On Wednesday night, I'm told Sir Keir called Mr Corbyn and said he had no plans to criticise him directly and would be "unfactional" in his response.
Fast forward to Thursday lunchtime, and the man who a year ago was Labour's candidate to be prime minister, found himself kicked out, for now at least, of the party he has been a member of since he was a teenager.
It was Mr Corbyn's response to the report - that anti-Semitism had been "dramatically overstated" by opponents - that prompted his suspension, and provoked this backlash from his supporters.
After a crushing election defeat and the dark cloud of anti-Semitism allegations, Sir Keir does have to define himself as profoundly different from his predecessor. But in so doing, there is plenty of scope for turbulence.
Mr Corbyn, MP for Islington North, led the Labour Party for five years until April - running as the party's candidate to be prime minister less than a year ago.
He was replaced by Sir Keir in April, but remained as a backbench MP, having been a member of the party for 55 years after joining as a teenager.
His time in charge of the party was dogged by allegations of anti-Semitic abuse and harassment by some of its supporters, especially on social media.
In May 2019, the Equalities and Human Rights Commission announced it would be investigating the party over its handling of the claims.
And on Thursday, it found Labour responsible for three breaches of the Equality Act:
- Political interference in anti-Semitism complaints
- Failure to provide adequate training to those handling anti-Semitism complaints
- Harassment, including the use of anti-Semitic tropes and suggesting that complaints of anti-Semitism were fake or smears
In his response to the EHRC's findings, Mr Corbyn said he had "acted to speed up, not hinder the process" of dealing with complaints, but the scale of anti-Semitism within Labour had been "dramatically overstated for political reasons by our opponents inside and outside the party".
Sir Keir said those who believed the issue of anti-Semitism in the party had been "exaggerated" should "be nowhere near the Labour Party".
Labour general secretary David Evans suspended Mr Corbyn, with the party saying this had happened "in light of his comments" and "his failure to retract them subsequently".
The suspension will remain in place while an internal investigation into his remarks is carried out, and Mr Corbyn has appealed to the party to reinstate him.
Sir Keir told the Today programme he was "deeply disappointed" in Mr Corbyn's response to the report which appeared to contradict what he planned to say, as he had told his predecessor about it the night before.
He added: "I'm in no doubt that Jeremy Corbyn and his team knew exactly what I was going to say in my response about, not only anti-Semitism, but about the denial and the arguments about exaggeration and it's just a factional fight."
- IS LABOUR ANTI-SEMITIC?: From July 2019: Panorama goes inside the anti-Semitism crisis gripping the Labour Party