One of Labour’s largest private donors has abandoned the party over his disgust at the leadership’s “failure” to deal with “most blatant acts of antisemitism” in its ranks.

Sir David Garrard, who has donated more than £1.5m to Labour under its three previous leaders, said he had watched with “dismay and foreboding” at how some of the party’s most senior figures have behaved over the past two years, and claimed that the party he had supported “no longer exists”.

The leadership is scrambling to quell the growing crisis over anti-Jewish sentiment in its movement, which has spiralled since Jeremy Corbyn had to apologise for defending an antisemitic mural in 2012.

Labour has also sought to distance itself from a string of pro-Corbyn Facebook groups, which contained hundreds of antisemitic and violent comments, including Holocaust denial and praise for Adolf Hitler, according to The Sunday Times.

It comes after key Corbyn ally Christine Shawcroft resigned from Labour’s ruling body on Saturday night over allegations of antisemitism, following pressure from MPs, including deputy leader Tom Watson

Sir David told The Observer that the he had quit the party as he believed the leadership had “supported and endorsed” antisemitism.

He said: “As one of the former leading political and financial supporters of the Labour party, of which I was a member for so many decades, I no longer feel any affinity with, or connection to, what it seems to have become.

“I have watched with dismay and foreboding the manner in which the leadership has, in my view, over the last two years, conducted itself. I consider that it has supported and endorsed the most blatant acts of antisemitism.

“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 antisemitic attacks that led to such unbearable consequences for innocent millions in the past.

“So there no longer exists a party which even pretends to maintain and promote the principles and the integrity of what always was, to me, the Labour party.”

His intervention came amid claims that 12 staffers for Jeremy Corbyn and shadow chancellor John McDonnell were members of 20 social media groups which contained more than 2,000 racist, antisemitic and abusive messages, The Sunday Times reported.

Labour MP Luciana Berger said she and her staff had reported vile antisemitic abuse to the police, including one email urging her to kill herself. The Jewish MP said she had been called “Judas”, “a venal piece of detritus”, a “Zionazi” and an “absolute parasite.”

A Labour source said these groups received hundreds of posts each day, the majority of which were ordinary messages about events or party policy.

Many of the staff concerned were either no longer active on Facebook or were unaware they were group members and had not seen the content.

A Labour 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 antisemitism in all its forms. Any complaints of antisemitism are taken extremely seriously.

“These are fully investigated in line with our rules and procedures and any appropriate disciplinary action taken.”

Momentum director, Ms Shawcroft, stepped down from Labour’s national executive committee (NEC), as she said her membership had become a “distraction” and “an excuse for endless intrusive media harassment of myself, my family and friends”.

Ms Shawcroft had already quit as chair of the party’s disputes panel after it emerged she had opposed the suspension of a council candidate accused of Holocaust denial.

Mr Corbyn faced protests outside parliament this week, as well as criticism from senior Jewish figures, that he had failed to adequately address antisemitism in the Labour movement.

In his Passover message, Mr Corbyn promised to be an “ally”, saying: “We in the Labour movement will never be complacent about antisemitism. We all need to do better.

“I am committed to ensuring the Labour Party is a welcoming and secure place for Jewish people.”