Gordon Brown has said Jeremy Corbyn “has got to change” to address concerns over anti-Semitism in the Labour party.
The former prime minister said the issue was a “running sore” that had to be dealt with immediately because it was “simply wrong”.
Mr Brown was appearing at the Edinburgh International Book Festival where he was asked if he thought the Labour leader was a fit and proper person to be prime minister.
Mr Corbyn and the party have been engulfed in controversy after failing to endorse the International Holocaust Remembrance Alliance’s (IHRA) definition of anti-Semitism in its entirety.
The Labour leader has also come under fire for attending a controversial event at a Palestinian Martyrs Cemetery in Tunis in 2014.
Mr Brown told the book festival audience: “Jeremy Corbyn has got to change.
“He cannot sustain particularly what he is saying about the international agreement on what we do in our attitudes to both the Holocaust and to Israel.
“I predict to you that’s going to change within a few weeks. I believe that it will change but even that will not be enough.
“You have got to show by your actions not simply by saying some words that you understand the deep hurt that has been caused. I’m very clear about that.”
Mr Brown added: “We have a problem in Britain, we have a problem not just with Islamophobia and not just with racism against the black community. We have a problem within the Labour party with anti-Semitism and it has got to be dealt with.
“Within a month, within a few days we have to approve the international recommendations about how we deal with questions about the Holocaust and it’s absolutely central to the to progress of a democratic society that is tolerant and liberal that a party like the Labour party comes out strongly against any anti-Semitism within the far right.
“I’m not going to predict whats going to happen within the Labour party… but this problem has got to be sorted out and it’s got to be sorted out immediately.”
Mr Brown continued: “This cannot keep going on as a running sore and it’s not because it’s an embarrassment, it is because it is simply wrong.
“The persecution that has been suffered by the Jewish community must never be forgotten. It is something that has got to be remembered every time we see vicious actions and discrimination and prejudice in different communities around the world.”