Jeremy Corbyn once met with a representative of the terrorist group, the Popular Front for the Liberation of Palestine – General Command, it has been revealed.
The PFLP-GC was responsible for a number of terror attacks in Israel in the 1970s and 80s, victims of which included children.
The meeting, revealed on Wednesday by The Golam Twitter account, took place in Beirut in 2011 while Mr Corbyn was part of a delegation to Lebanon.
The trip was organised by the PRC, a group with alleged links to Hamas.
The UK designated the PFLP-GC a proscribed terrorist organisation in 2014. It was described as “a left-wing nationalist Palestinian militant organisation".
The group’s aim is the destruction of Israel and is opposed to any negotiation with Israel.
The PFLP-GC was responsible for a number of brutal attacks against Israeli civilians.
These include the May 1970 Avivim school bus massacre in which 12 people were murdered, including nine children, when two rocket-propelled grenades were fired at a school bus on its way to school.
A US State Department report from last year said: "Since the early 1990s, the group has primarily focused on supporting Hizballah's attacks against Israel, training members of other Palestinian terrorist groups, and smuggling weapons."
The JC has approached Labour for comment.
