Labour has suspended former London mayor Ken Livingstone over a series of comments branded "vile" and "offensive" and verging on anti-Semitism by the party's deputy leader Tom Watson.
Here we look at some of the key questions raised:
Why was Ken Livingstone suspended?
In a series of broadcast interviews on Thursday, Livingstone defended MP Naz Shah, who had herself been suspended by the party after posting offensive comments on social media before she was elected in Bradford West.
She had shared an image on Facebook of the outline of Israel superimposed on a map of the US, under the heading 'Solution for Israel-Palestine Conflict - Relocate Israel into United States'. She added the comment "problem solved", and said she would pass on the suggestion to David Cameron and Barack Obama.
Livingstone told BBC Radio London: "It's completely over the top but it's not anti-Semitism. Let's remember when Hitler won his election in 1932, his policy then was that Jews should be moved to Israel. He was supporting Zionism - this before he went mad and ended up killing six million Jews."
The reference to Nazi leader Adolf Hitler provoked fury within Labour ranks, with shadow cabinet members and London mayoral candidate Sadiq Khan demanding his suspension.
Why did Livingstone make these claims?
The former London mayor is relying on research by American writer Lenni Brenner. His 1983 book Zionism In The Age Of Dictators was an expose of Zionist collaboration with Hitler. Livingstone met Brenner in the 1980s, when the Labour politician was leader of the Greater London Council, and has said he will use the author's work to fight his case with the party authorities.
But Brenner's findings have been disputed, with British historian Roger Moorhouse saying that although there was a "Zionist arrangement of sorts with Hitler's Germany", in that Jews could pay a fee of £1,000 to emigrate to British Mandated Palestine, "to conclude that Hitler therefore 'supported Zionism' is not only historically inaccurate, it is historically illiterate".
Who is Lenni Brenner?
He is an American activist, writer and historian with a long history of links to political causes. Born into an orthodox Jewish family, he became a civil rights activist at 15, involved in campaigns including the Congress of Racial Equality, the National Association for Irish Justice - a US organisation with links to the Catholic civil rights movement in Northern Ireland - and the Vietnam anti-war movement.
What will happen next in the row?
The dispute has already proved deeply embarrassing for Labour, demonstrated by the extraordinary clash between Livingstone and Labour MP John Mann - who called him a "Nazi apologist" - outside the TV studios in Westminster.
Livingstone's fate will be decided by the ruling National Executive Committee (NEC) which will investigate whether he brought the party into disrepute.
Deputy leader Tom Watson said "no one is in any doubt that Ken Livingstone's behaviour yesterday has let down the Labour Party" but Livingstone will fight his corner, claiming "it's hard for somebody to decide to suspend me from the party here when all this was there 30 years ago in the public domain and nobody raised a peep".
What does this mean for Jeremy Corbyn and Labour now?
The Labour leader has insisted there is no "crisis" but he has been criticised from both inside and outside the party over this response to allegations of anti-Semitism.
Watson said both he and Corbyn had been meeting Jewish community leaders to consider whether the party's structures could be improved to make sure there is a "zero-tolerance approach to anti-Semitism". That could mean changes to Labour's rules and procedures.