Keir Starmer has described a damning report of anti-Semitism in Labour as a "day of shame" for the party - and said people who deny the problem should be "nowhere near the party".
The Labour leader said he was "truly sorry" for the pain and grief caused to the Jewish community and said the party had failed its members and the British public.
Mr Starmer said the report by the Equality and Human Rights Commission was "hard to read" as he vowed to accept all its recommendations for ridding Labour of anti-Jewish hate.
But he repeatedly dodged questions on whether he would take action against former Labour leader Jeremy Corbyn.
The long-awaited report by the equalities watchdog delivered a damning verdict on Thursday on breaches of equalities laws by Labour relating to anti-Semitism.
The EHRC found that Labour had committed unlawful acts, including political interference in complaints and harassment.
The EHRC's interim chairwoman Caroline Waters said there had been "inexcusable" failures which "appeared to be a result of a lack of willingness to tackle anti-Semitism rather than an inability to do so".
Mr Starmer told a press conference: "I found this report hard to read and it is a day of shame for the Labour Party.
"We have failed Jewish people, our members, our supporters and the British public.
"And so on behalf of the Labour Party: I am truly sorry for all the pain and grief that has been caused."
He said recognised "how painful" the findings are for Jewish people, Labour members and whistleblowers.
"So let me be clear, I hear you and I can promise you this: I will act. Never again will Labour let you down," he said.
"The Labour Party I lead accepts this report in full and without qualification, we will implement all the recommendations and we will implement them in full. That process starts today."
Mr Starmer said anyone who says anti-Semitism within the party has been exaggerated "should be nowhere near" Labour, in a thinly-veiled jibe at his predecessor.
The Labour leader said: "If - after all the pain, all the grief, and all the evidence in this report - there are still those who think there's no problem with anti-Semitism in the Labour Party, that it's all exaggerated, or a factional attack, then, frankly, you are part of the problem too.
"And you should be nowhere near the Labour Party either."
Mr Corbyn responded to the EHRC report on Thursday, saying the issue was "dramatically overstated".
But Mr Starmer repeatedly dodged questions on whether Mr Corbyn should be kicked out.