John McDonnell has insisted there is "no room" in Labour for anti-Semites after the latest allegations of prejudice against Jews.
The shadow chancellor said Labour would not tolerate "racism in any form" after allegations anti-Semitic slurs were used in a Facebook group of which Jeremy Corbyn had been a member.
Mr McDonnell said any member engaged in anti-Semitic behaviour would be disciplined.
Mr Corbyn has admitted having been a member of the controversial private Facebook group about Palestine but denied ever seeing anti-Semitic posts.
His son, Seb Corbyn, an aide to Mr McDonnell, was also reported to have been in the group.
Mr McDonnell said both Mr Corbyn and his son were added by others to the Facebook group.
He said: "Jeremy participated in the group in making sure his views were expressed about the need for a peace process to tackle the problems of Palestine and rebuild the relationship between Palestine and Israel as best it could."
But he said "you cannot control what others say" and Mr Corbyn withdrew from the group when some of the content was pointed out to him.
Mr McDonnell said: "If there is any Labour Party member that is engaged in anti-Semitic behaviour, they will be disciplined and there will be no room for them in our party."
Shadow women and equalities minister Dawn Butler said Labour's policies on the issue were aimed at ensuring it was "more robust" than any other party.
Mr Corbyn told the Press Association on Thursday: "I was joined on to that group without knowing it in probably about 2013/14.
"I removed myself from the group in 2015. I replied by Facebook message to a couple of things about a suggestion on the vote on recognising Palestine, which I supported, and inviting a doctor to speak at an event.
"I have never trawled through the whole group. I have never read all the messages on it. I have removed myself from it.
"Obviously, any anti-Semitic comment is wrong. Any anti-Semitism in any form is wrong."
The Campaign Against Anti-Semitism published screenshots of messages said to have been posted on the Palestine Live group.
Among the comments exchanged are claims about Israeli involvement in the 9/11 terrorist attacks, as well as links to neo-Nazi group postings.
Former chief rabbi Lord Sacks has called on the Labour leadership to do more to combat anti-Semitism before he would even contemplate talks on the issue.
He told The Jewish News: "I would want to see clearer signs of resolute action by a party and its leader before I would even sit down with them full stop."