Labour has launched an investigation into claims party members posted anti-Semitic comments on a closed Facebook group dedicated to Palestinian rights.
An anti-Semitism group said evidence gathered by the blogger David Collier suggested Jeremy Corbyn was said to be among members of Palestine Live until just before he became leader in 2015.
Labour said there was no suggestion Mr Corbyn had made any anti-Semitic posts.
The Labour leader was unaware of the alleged comments, the spokesman added.
Exchanges by members of the closed Facebook group were said to have included discussions of conspiracy myths about the Rothschild family and supposed Israeli involvement in the 9/11 terrorist attacks, as well as links to material produced by neo-Nazi groups.
The Campaign Against Anti-Semitism said there was "no conceivable justification" for Mr Corbyn's alleged involvement in the group and that it would be filing a disciplinary complaint to the party against him.
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Mr Corbyn's spokesman said: "Jeremy and the Labour Party are implacably opposed to all forms of anti-Semitism and will take whatever necessary action to stamp it out in the Labour Party.
"I don't think anyone is suggesting that anything that Jeremy has written in any Facebook group or anywhere else constitutes anti-Semitic comments.
"As I understand it, there were thousands of members of this group. As anyone knows, in social media all sorts of things are posted which often others participating aren't even aware of.
"It is repugnant if there are anti-Semitic posts – and I gather there are. If they involve anyone to do with the Labour Party then investigation and disciplinary action will be taken."
Mr Collier is a British businessman who lived in Israel for nearly 20 years and whose blog is dedicated to "researching anti-Semitism inside anti-Zionist activity".
An internal inquiry in 2016 found that Labour was not overrun by anti-Semitism or other forms of racism but that there was an occasionally "toxic atmosphere" within the party.
The probe, which led to Labour adopting tougher rules on anti-Semitism but which was criticised as a whitewash by Jewish leaders and some MPs, was triggered by a number of high-profile incidents.
Former London Mayor Ken Livingstone remains suspended from the party after he refused to apologise for statements he made about Adolf Hitler and Zionism.