Gordon Brown has called on Labour to support the International Holocaust Remembrance Alliance’s definition of antisemitism “unanimously, unequivocally and immediately”, saying he “cannot be silent on an issue so fundamental to our future”.

The former prime minister was speaking at the Jewish Labour Movement’s annual conference, two days before the party’s ruling national executive committee (NEC) has to decide whether to adopt the code in full with all its examples, at a time of internal turmoil over the issue.

He conceded the party had a problem with antisemitism and needed to deal urgently with complaints that have clogged up its disciplinary procedures, arguing anti-Jewish sentiment was “a problem of the conspiracy-theory left”.

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Brown said he was not appearing as “some sort of backseat driver”, but he had to speak out about antisemitism because equality and solidarity was “what the Labour party is all about, or should be all about”.

The speech did not mention the Labour leader, Jeremy Corbyn, by name, or allude to him, and was clearly aimed at the crunch NEC meeting on Tuesday, which will debate whether to adopt the IHRA code with all its examples, a stance now supported by the leadership after a summer of controversy.

Labour’s leadership had originally suggested it would adopt the code in full, but not all 11 attached examples. But it has gradually retreated from that position and hopes the NEC will follow suit.

Brown spoke about Labour’s history of fighting antisemitism and racism. “It’s very clear we made a promise to the Jewish community that we must honour,” he said.

“You will never walk alone and we will never walk by on the other side. We will not neglect you, we will not forget you, we will not desert you. If your voice is silenced, we will lend you our voice.”

He said he had read the IHRA document and it was clear that “criticism of Israel is not to be taken as antisemitism”, and “you can criticise the government and you can support the Palestinian cause for a two-state solution”.

He added that he supported the creation and existence of the state of Israel and a Palestinian state in the Middle East.

Earlier, Jonathan Sacks, the former chief rabbi, repeated his criticism of Corbyn, saying that he had “to express clear remorse” and “repent and recant” after a film had emerged of the Labour MP in 2013 angrily criticising a group of “Zionists” who he said “don’t understand English irony”.

Sacks told the BBC’s Andrew Marr Show that Corbyn was “as great a danger as Enoch Powell was”, reiterating a controversial comparison with the far-right politican he had first made last week.

Labour’s shadow chancellor, John McDonnell, responded by saying he found Sacks’s remarks “quite distressing” and said that the former chief rabbi had been “brutally honest”. But he defended Corbyn, saying: “I just think he’s [Sacks] got it wrong.”

The Labour MP and close ally of Corbyn said Sacks had “really misunderstood Jeremy – his whole life has been devoted to anti-racism, to peace and justice”. He called on Sacks to “come and sit down with Jeremy” to discuss tackling antisemitism, although the former chief rabbi said earlier he would not do so without an apology first.

McDonnell went on to refer to Tuesday’s NEC meeting, saying: “All sides will be satisfied with the proposals that will be discussed.” He indicated that while the IHRA definition would be accepted in full, there there would also be some sort of additional statement on free speech.

“I think acceptance overall, and I think also the commitment to freedom of speech, and yes, a recognition of the rights of Palestinians,” McDonnell said.

“From what I’ve heard, it will be resolved, and there will be a balance about acceptance, as people want, but also, exactly about what others have said as well - Lord Sacks himself - that freedom of speech is important as well.”