Labour has launched a disciplinary investigation into claims party members posted anti-Semitic comments on a closed Facebook group dedicated to Palestinian rights.
The Campaign Against Anti-Semitism said evidence gathered by the blogger David Collier suggested Jeremy Corbyn was said to be among the members of the Palestine Live group until shortly before he became leader in 2015.
Exchanges by group members 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.
A spokesman for the Labour leader said there was no suggestion he had made any anti-Semitic postings and that he had been unaware of the alleged comments.
However the Campaign Against Anti-Semitism said there was "no conceivable justification" for Mr Corbyn's involvement in the group and that it would be filing a disciplinary complaint to the party against him.
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."
Joseph Glasman of the Campaign Against Anti-Semitism, said: "There is no conceivable justification for Jeremy Corbyn's participation in this group.
"One of Mr Corbyn's slogans is 'standing up, not standing by', but in this case he has not stood up but instead he actively joined in."