In a carefully delivered speech to a Momentum-organised event on Friday evening, which he said he was reading out from pre-written script to avoid any controversy, Mr McDonnell had told activists: “I’m going to read much of what I’ll say because we have to be careful in our language at the moment. We don’t want to give any hostages to fortune to any who would use our words, distort them and use them against the left.
“It’s true, as well, that political opponents have attacked us over the issue of antisemitism.
“Time and time again the Tories raised this in parliament, but without once criticising the racist and antisemitic behaviour of Boris Johnson their own leader. I even had antisemitism raised in Treasury questions by the Tories.
“But this does not mean that the evidence of antisemitism in our party doesn’t and didn’t exist, or was trumped up, and it doesn’t excuse not acknowledging its existence and stopping it.
“But the acknowledgement made by us all of the horrendous nature of antisemitism and its existence in our party and our society is something that should bring us together not divide us… to ensure we fight against this evil.”
Mr McDonnell has a history of speaking out over antisemitsm in Labour, while also siding with groups who have been seen to have been at the heart of the problem under Mr Corbyn.
He once met Jewish Labour activists in a bid to ease tensions under Mr Corbyn.
But at the same time he has remained president of the Labour Representation Committee, whose executive includes Jackie Walker, who was expelled from the party over allegations involving antisemitism.