Gordon Brown will today urge the Jewish Labour Movement not to quit the party.
The group is deciding whether to end its 99-year affiliation with Labour over anti-Semitism.
The former PM has helped the JLM recruit new members, with numbers up tenfold to 2,750 in three years.
Mr Brown said: “We must never tolerate intolerance. Solidarity means standing up with those who are under attack and that is why I am joining as an affiliated member.
“I urge all my colleagues to do the same.
“We will never allow evil to triumph over good and neither will the British people.”
It comes after Countdown star Rachel Riley admitted she would vote Tory due to "Labour's anti-Semitism".
The Channel 4 TV host said she believed the Labour leader was "allowing" the issue to fester in the party's ranks.
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Asked if she thought that the Labour leader was himself anti-Semitic, Ms Riley said: "I think it’s a pointless argument.
The Countdown star has faced online abuse for speaking out against alleged anti-Semitism in the Labour Party, with opponents calling on a boycott of Riley.
The 33-year-old continued: "I couldn’t give a monkey’s if he is anti-Semitic or not himself, because the environment he seemingly supports and is encouraging, the stuff that people are claiming to do in his name, is vile.
"So, for me, whether he says he’s an anti-Semite or not, he’s allowing this to happen. He’s condoning it, in my view," she added.
