One of Labour's biggest private donors has launched a bitter attack on the party's failure to deal with anti-Semitism under Jeremy Corbyn.
Sir David Garrard, who has donated around £1.5 million since 2003, said he had now left the party having seen it fail to respond to "the most blatant acts of anti-Semitism".
His intervention came as The Sunday Times reported that 12 senior staff working for Jeremy Corbyn and shadow chancellor John McDonnell were members of social media groups containing anti-Semitic and violent comments.
The paper said an investigation into 20 of the biggest pro-Corbyn Facebook groups - numbering around 400,000 members - had uncovered routine attacks on Jewish people, including Holocaust denial.
Working with whistleblowers who were able to gain access to restricted membership groups, it said that it had uncovered more than 2,000 racist, anti-Semitic, misogynistic, violent and abusive messages.
A Labour source said such sites routinely received hundreds of postings a day, most of which were perfectly innocent messages about party policies or events.
Many of the staff concerned were either no longer active on Facebook or were unaware they were members of these and had not seen the content highlighted by the paper.
A Labour Party spokesman said: "These groups are not run by the Labour Party or officially connected to the party in any way.
"The Labour Party is committed to challenging and campaigning against anti-Semitism in all its forms. Any complaints of anti-Semitism are taken extremely seriously.
"These are fully investigated in line with our rules and procedures and any appropriate disciplinary action taken."
However the Jewish Labour MP Luciana Berger - who is a regular target for abuse - said she and her staff had gone to the police over the abuse they had suffered from left-wingers, including one email urging her to kill herself.
"Where people indulge in illegal racist activity I will always use the full force of the law to pursue a prosecution," she said in an article for The Sunday Times.
"I will continue to do that even when they are people from the left."
Meanwhile Sir David, who donated to Labour under its previous three leaders, spoke of his disillusionment with the party under Jeremy Corbyn.
"I have watched with dismay and foreboding the manner in which the leadership has, in my view, over the last two years, conducted itself" he told The Observer.
"I consider that it has supported and endorsed the most blatant acts of anti-Semitism.
"And yet it has failed to expel many of those who have engaged in the grossest derogatory fantasies about Jewish/Zionist conspiracies - and Jewish characterisations and accusations which conjure up the very kind of anti-Semitic attacks that led to such unbearable consequences for innocent millions in the past."
The attacks came as the senior Labour official at the centre of another row over anti-Semitism row, said she was stepping down from the party's ruling body.
Christine Shawcroft, a left wing supporter of Jeremy Corbyn said her presence on the national executive committee (NEC) had become a "distraction" and was resigning with immediate effect.
Ms Shawcroft had already quit as chairwoman of the party's disputes panel after it emerged she had opposed the suspension of a council candidate accused of Holocaust denial.
She had been under intense pressure to give up her seat on the NEC as well, with many MPs furious after she claimed in a Facebook posting that the row was being "stirred up to attack Jeremy, as we all know".
She said: "It is clear that my continued membership of the NEC has become a distraction for the party and an excuse for endless intrusive media harassment of myself, my family and friends."
It is expected that under party rules her seat will now go to comedian Eddie Izzard - the next most popular candidate at the last NEC election.