The Labour Party was "responsible for unlawful acts" of anti-Semitic discrimination and harassment, a report by the Equality and Human Rights Commission has found.
The 16-month investigation into Labour regarding anti-Semitism under the leadership of Jeremy Corbyn also said the party broke equality laws.
The 130-page report said it found "significant failings in the way the Labour Party has handled anti-Semitism complaints over the last four years".
The Jewish Labour Movement said blame for the "sordid, disgraceful chapter" in the party's history "lies firmly with those who held positions of leadership".
Pointing blame at Mr Corbyn, the report said: "It is hard not to conclude that antisemitism within the Labour Party could have been tackled more effectively if the leadership had chosen to do so."
Responding to the report, Mr Corbyn said Jewish Labour members were right to expect the party to deal with anti-Semitism "and I regret that it took longer to deliver that change than it should", but he stopped short of apologising.
Mr Corbyn added: "The scale of the problem was also dramatically overstated for political reasons by our opponents inside and outside the party".
The backbench MP said "processes for handling complaints were not fit for purpose" when he became leader in 2015 and fixing the problem "stalled by an obstructive party bureaucracy".
But he said that from 2018 "substantial improvements" were made, "making it much easier and swifter to remove anti-Semites".
Listen to the ITV News Politics Podcast:
"My team acted to speed up, not hinder the process," he added.
The statement went on: " My sincere hope is that relations with Jewish communities can be rebuilt and those fears overcome. While I do not accept all of its findings, I trust its recommendations will be swiftly implemented to help move on from this period."
Current Labour Leader Sir Keir Starmer, responding to the report, admitted the findings represented a "day of shame" for the party and said he was "truly sorry".
But he appeared to reject the suggestion that Mr Corbyn should be expelled from the party, saying "the report does not make any individual findings about Mr Corbyn, it has strong findings about a collective failure of leadership".
Sir Keir said he would look at Mr Corbyn's response to the report later, but added: "Those that deny there's a problem are part of the problem."
The report, which was launched in May 2019, was commissioned to investigate allegations of anti-Semitism in the party and how it responded to complaints.
It said there were "serious failings" in the way complaints were handled until at least 2018.
"Specific examples of harassment, discrimination and political interference" were found, but the report also noted "a lack of leadership within the Labour Party on these issues", which it said was "hard to reconcile with its stated commitment to a zero-tolerance approach to anti-Semitism".
It said: "The Labour Party must live up to this commitment and acknowledge the impact that multiple investigations and years of failing to tackle anti-Semitism has had on Jewish people."
Watch Labour Leader Keir Starmer respond to the EHRC report:
Caroline Waters, Interim Chair of the Equality and Human Rights Commission, said the party's "approach and leadership to tackling antisemitism was insufficient".
"This is inexcusable and appeared to be a result of a lack of willingness to tackle antisemitism rather than an inability to do so."
Sir Keir, since taking over from Mr Corbyn, has committed to a zero tolerance approach to anti-Semitic discrimination in the party.
But the watchdog found that the lack of training for people handling anti-Semitism complaints indirectly discriminated against Jewish members until August 2020, by which time Sir Keir was running the party.
Sir Keir apoligised, saying Labour had "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."
In a statement, Jewish Labour Movement said: "Since 2015, we have consistently warned the Labour Party about a deepening casual culture of anti-Jewish racism, that saw Jewish Labour members and activists harassed and discriminated against.
"Instead of listening to our growing concerns over the scale of the challenge, we were told that this racism was imagined, fabricated for factional advantage or intended to silence debate.
"Today's report confirms that our voices were marginalised and our members victimised."
Former Labour MP John Mann, who serves as an advisor to the government on anti-Semitism, wrote on Twitter: "The moment of greatest shame in the history of the Labour Party.
"And to think how many said it was all made up and exaggerated. Which amongst them will stand up and say that I am truly sorry?"
The report stated: "We found that the Labour Party's response to anti-Semitism complaints has been inconsistent, poor and not transparent, in terms of the process used, reasons for decisions, record-keeping, delay and failures to communicate with complainants.
"Some complaints were unjustifiably not investigated at all."
Under Mr Corbyn, the party "at best, did not do enough to prevent antisemitism and, at worst, could be seen to accept it," the report added.
The report also found "evidence of political interference in the handling of anti-Semitism complaints throughout the period of the investigation".
The party has been served with an unlawful act notice and has been given until December 10 to draft an action plan to implement the report's recommendations, which is legally enforceable by the courts if not fulfilled.
Sir Keir promised to implement the EHRC recommendations in full.
"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."
The EHRC report can be read in full here.