Sir Keir Starmer’s judgement came under fire over the delay in withdrawing support for Rochdale by-election candidate Azhar Ali after his anti-Israel rant.
Mr Ali was recorded suggesting in a meeting of the Lancashire Labour Party that Israel had allowed the October 7 Hamas slaughter of more than 1,200 citizens as a pretext to invade Gaza.
The story was broken by the Mail on Sunday but Labour stuck by Mr Ali despite the deeply controversial remarks.
Shadow minister Nick Thomas-Symonds was even sent out onto the airwaves to defend the party’s stance.
But on Monday evening, Labour finally cut adrift Mr Ali who is understood to have also been suspended from the party pending an investigation.
A Labour Party spokesperson said: “Following new information about further comments made by Azhar Ali coming to light today, the Labour party has withdrawn its support for Azhar Ali as our candidate in the Rochdale by-election.”
Speaking as he campaigned in Wellingborough on Tuesday, Sir Keir said: "Certain information came to light over the weekend in relation to the candidate. There was a fulsome apology. Further information came to light yesterday calling for decisive action, so I took decisive action.
"It is a huge thing to withdraw support for a Labour candidate during the course of a by-election.
"It's a tough decision, a necessary decision, but when I say the Labour Party has changed under my leadership I mean it."
But London minister Greg Hands accused Sir Keir of taking too long to “get off the fence”.
He added: “It should have been obvious that Azhar Ali couldn’t be an MP for a Party aspiring to be in government. “If Sir Keir can’t take the tough decisions as Labour Leader, he can’t be up to the job as Prime Minister.”
Labour MPs also feel there is a "disparity in treatment" within the party about antisemitism allegations, the lawyer of the Forde Report has said.
Martin Forde KC led the 2022 report that found factional disputes between the party's Right and Left under former leader Jeremy Corbyn, and that both sides had weaponised antisemitism.
He told BBC Radio 4's Today programme that Labour's decision to withdraw support from Mr Ali 36 hours after allegations of antisemitism emerged "appears pretty shambolic".
Mr Forde said: "If you want a fair and transparent system then it has to deal with people consistently, and I'm aware from discussions with some of the MPs within the party - who might be described as Left-leaning - that they feel that when it comes to disciplinary action taken against them then things move rather slowly, but if you're in the Right faction of the party, as it were, then things are dealt with either more leniently or more swiftly.
"Now that's the perception, I can't quantify it, but I do think it's something that leadership should be concerned to, in a away, dilute, or if it is in fact the case, they need to give reassurance to members of the voting public and to their members that people will be treated fairly."
He said Labour MPs Kate Osamor and Diane Abbott both apologised immediately after they were accused of antisemitism, but "things seemed to drag on in terms of disciplining certain elements of the party, and be dealt with swiftly in others".
Labour faced intense pressure since Mr Ali’s remarks surfaced, with widespread condemnation from party members and political rivals.
Mr Ali is alleged to have told a meeting of the Lancashire Labour Party: "The Egyptians are saying that they warned Israel 10 days earlier... Americans warned them a day before [that] there's something happening...
"They deliberately took the security off, they allowed... that massacre that gives them the green light to do whatever they bloody want."
After the Mail on Sunday published details of the recording, Mr Ali issued an apology stating that October 7 had been "the greatest loss of Jewish life in a single day since the Holocaust, and Jews in the UK and across the world are living in fear of rising antisemitism".
Labour stood by him but then it emerged that at the party meeting in Lancashire Mr Ali apparently also blamed Jewish media figures for fuelling criticism against a pro-Palestinian Labour MP.
The Daily Mail published a fuller recording, allegedly of Mr Ali, blaming "people in the media from certain Jewish quarters" for the suspension of Andy McDonald from the party.
Leftwinger Mr McDonald, a key ally of Mr Corbyn, was suspended last year after saying: "We will not rest until we have justice. Until all people, Israelis and Palestinians, between the river and the sea, can live in peaceful liberty."
A spokesperson for Campaign Against Antisemitism said: “Sir Keir Starmer has blotted an otherwise fairly admirable copy book and given the public reason to doubt the earnestness of his promise to tear antisemitism out ‘by its roots’ in Labour.
“People will have to judge for themselves whether the additional reported comments by Azhar Ali are really any worse than the comments that had already been reported.
“Rather than appearing as a principled decision, Labour’s withdrawal of support for its candidate at this late stage just looks as expedient as the failed attempt to defend him.
“It is the worst of all worlds for Labour.”
It is too late for Labour to withdraw Mr Ali as their candidate and replace him with someone else as the deadline passed on February 2.
Also running in Rochdale are former Labour MP Simon Danczuk, now the Reform Party candidate, and George Galloway, of the Workers Party of Britain, who is campaigning against Labour's stance on Gaza.
About 20 per cent of the electorate and 30 per cent of the population of the town are Asian, with polls nationally suggesting Labour's vote could be hit by Asian people unhappy with the party over Palestine and its perceived support for Israel.