John McDonnell claimed Israel was attempting to carry out “genocide” against the Palestinians and he shared a platform with a woman who described Jews as the “chief financiers” of the slave trade, The Telegraph can reveal.

The Board of Deputies of British Jews has demanded that the shadow chancellor apologise to the Jewish community for his “wild claims”, as a previously unseen video emerged in which he accused the Israeli government of ethnic cleansing.

However, Labour stood by Mr McDonnell’s comments and suggested that they were “justifiable” in view of the “brutal attacks on the Palestinian men, women and children of Gaza”.

A spokesman said: “John takes pride in and stands by his track record... and will always stand up for the victims of such disproportionate violence.”

In November 2012, addressing a far-Left rally organised by Unite the Resistance, Mr McDonnell said: “Nobody can speak without expressing some form of solidarity with the people of Gaza... as the children are murdered and the bombs are flying from Israel.

“I think it’s absolutely critical now that we use every platform we can to expose what’s going on, which is effectively an attempt at genocide against the Palestinians.”