Jeremy Corbyn addressed a rally attended by hundreds of members of Al-Muhajiroun, the extremist group linked to London Bridge attacker Khuram Butt.
The rally took place in May 2002 and the Labour leader gave a speech whilst supporters of Hezbollah and former terrorists listened, according to conservative political website Guido Fawkes.
The rally was attended by thousands of people, and there is no suggestion that either Corbyn or any member of his team had any links to the group.
The left-wing Weekly Worker newspaper, which produced a report on the 2002 event, claimed there were up to 300 members of Al-Muhajiroun at the rally.
It also claimed some were dressed up as suicide bombers and chants were heard of “gas, gas Tel Aviv" and the extremist group held placards which said "Palestine is Muslim."
Al-Muhijaroun, founded by jailed preacher Anjem Choudary, has previously been linked to the 7/7 bombings and the murder of Lee Rigby, and it returned to prominence this week when it emerged that Khuram Butt, one of the three men who carried out the London Bridge terror attack, was a supporter.
Corbyn's spokesperson James Schneider didn't deny Corbyn addressed the rally, but said the Labour leader had no control over the attendees.
"Jeremy addressed a broad-based rally in support of Palestinian rights," Schneider told talkRADIO.
"It was a public event and he was in no way responsible for the views of all of the thousands of attendees. Jeremy condemns Al-Muhajiroun in the strongest possible terms."