The Labour leadership has faced a backlash over the latest anti-Semitism row to hit the party, with a major donor claiming a greater effort was needed to "stop the rot" in the party.
The party has suspended MP Naz Shah pending an investigation over offensive messages on social media.
The West Bradford MP was stripped of the parliamentary whip and barred from party activity pending an investigation of her behaviour - which David Cameron branded racist.
Opposition leader Jeremy Corbyn had initially delivered only a reprimand, arguing that while the comments - made before Ms Shah was an MP - were "offensive and unacceptable", Ms Shah did not hold discriminatory views.
But under pressure from senior Labour figures to take disciplinary action amid mounting concern about anti-Semitism within the ranks - and with the Prime Minister joining calls for her suspension - it was announced Ms Shah had been excluded.
Ms Shah, who quit her role as a parliamentary assistant to shadow chancellor John McDonnell on Tuesday, told MPs she deeply regretted the hurt caused by the posts and wanted to work with Jewish groups to bolster understanding.
In a 2014 Facebook post, she shared a graphic of Israel's outline superimposed onto a map of the US under the headline "Solution for Israel-Palestine Conflict - Relocate Israel into United States", with the comment: "Problem solved".
The Guido Fawkes website - which published the post - also pointed to another made before Ms Shah was an MP, which used the hashtag #IsraelApartheid above a quote saying "Never forget that everything Hitler did in Germany was legal".
Labour donor David Abrahams told Jewish News: "I have been appalled by the growth of anti-Semitism in the party. This is a plague that has to be stamped out. Jews and others with values and principles need to work together within the movement to stop the rot."
Labour peer Lord Levy, a former fundraiser for the party, told BBC's Newsnight: "Unfortunately I have to say that I do think that it's a serious problem. The lack of sensitivity when an MP talks about transportation of the largest Jewish community in the world, I think it just shows such ignorance.
"The tweets and comments that she made on Hitler, I just begin to scratch my head in despair as to how people like this can enter our Parliament with such a lack of knowledge, such a lack of discretion and such a lack of sensitivity."
He added that it had "taken too long" for the leadership to act in Ms Shah's case and Mr Corbyn's team had "dithered".
He said "every party needs to look very carefully in their cupboards as to what is going on on anti-Semitism at the moment".
Rabbi Baroness Neuberger, a crossbench peer, told Newsnight: "I think Labour does have a problem with anti-Semitism. That's not to say that other parties haven't got problems with anti-Semitism, or that it isn't elsewhere.
"But Labour has a very particular problem, and a particular problem at the moment.
"I think the Naz Shah case illustrates that, but more than anything else, Oxford University Labour Club."
The chairman of the Oxford University Labour Club recently resigned, saying a "large proportion" of its members had "some kind of problem with Jews" and the party has launched an investigation.
Lady Neuberger added: "I was brought up in the Labour Party, my parents would be turning in their graves."
Labour MP Ian Austin, whose Czech Jewish father fled Nazi persecution, said: " What a mess. Aged 16, I joined the Labour Party to fight racism. I would never have thought we'd have ended up here."
Ms Shah's suspension was confirmed by Labour shortly after she apologised to MPs for her posts.
"Jeremy Corbyn and Naz Shah have mutually agreed that she is administratively suspended from the Labour Party by the general secretary," a party spokesman said.
"Pending investigation, she is unable to take part in any party activity and the whip is removed."
Ms Shah told the Commons: "Anti-Semitism is racism, full stop. As an MP I will do everything in my power to build relations between Muslims, Jews and people of different faiths and none."
Ms Shah wrote in Jewish News that she wished to make an "unequivocal apology for statements and ideas that I have foolishly endorsed in the past".
"The manner and tone of what I wrote in haste is not excusable. With the understanding of the issues I have now I would never have posted them. I have to own up to the fact that ignorance is not a defence."
But Ken Livingstone - the former London Mayor who is a close ally of Mr Corbyn - said Ms Shah's remarks were not anti-Semitic and the suspension was unnecessary.
He told LBC Radio: "What we have at the moment is a lot of people making a big issue about anti-Semitism in the Labour Party. In 47 years I have never heard anyone say anything anti-Semitic.
"We expelled a couple of people from the Labour Party early on for saying things that could clearly be interpreted as anti-Semitic.
"This is not that; this is an over-the-top comment about the horrendous conflict between the Israelis and the Palestinians."