There have been many explanations for the rapid spread of antisemitic utterances within the British left.

For some, the explanation is that it is ideologically ingrained since the birth of socialism; for others, sheer ignorance about Jewish history exacerbated by social media; for still others, an indifference to the Jews per se, that Jews are unimportant in the grand scheme in working for the greater good.

But a central factor is the visceral opposition to Zionism — regarded as racism and solely responsible for the exodus of Arabs from Palestine in 1948.

Many committed Labour figures started off on the far left but eventually moved away.

Len McCluskey was close to the Militant Tendency, Ken Loach was associated with the Socialist Labour League while Seumas Milne worked with the Communist party faction Straight Left.

Views, however, often evolved to meet a changing situation. Indeed, Sir Alfred Sherman, a Jew from Hackney, fought as a Communist in the Spanish Civil War in the 1930s but remarkably ended up providing intellectual foundations for Thatcherism in the 1970s.

Seumas Milne, however, widely regarded as Mr Corbyn’s political mentor, has not mellowed with age but has carried his political philosophy and hardcore anti-Zionism into the heart of the Labour Party in 2019.

In the inter-war years, many Jews belonged to the Communist Party to fight fascism and to create a better world.

In 1939 they were suddenly asked to place their belief in the Nazi-Soviet pact and Stalin’s good judgment. They were asked to turn a blind eye to the millions of Polish Jews trapped in Hitler’s latest fiefdom so that the Soviet Union might benefit.

Communist Jews such as Eric Hobsbawm and Ivor Montagu enthusiastically went along with this — even though it was fast becoming a matter of life and death for many Jews. There were no qualms when the Soviets handed over 1,000 people, including 300 Jews, to the Gestapo at the border at Brest-Litovsk in 1940. Many changed their place of residence from the gulag to the concentration camp.

Many British Jews subsequently turned their back on Communism. The murder of the Yiddish writers, the Slansky trial in 1952 and the false accusation in 1953 that a cabal of Jewish doctors was planning to poison the leaders of the USSR burst the bubble and Jews deserted the party in droves. The Soviet invasion of Hungary in 1956 further caused an exodus of loyalists from the party.

In the early 1960s, the continual reports of antisemitism in the USSR, the publication of crude caricatures in its press and the equating of Israeli Zionists with German Nazis — all before the conquest of the West Bank and the settlement drive — was the subject of internal discussion within the British Communist Party.

Dissent finally surfaced when the party condemned the Soviet invasion of Czechoslovakia and the crushing of “socialism with a human face” in 1968.

Such condemnation of the Soviet Union catalysed schisms within the party. The Soviet stand-off with China spawned several Maoist groups. By the end of the 1970s, the party was riven with factions.

The Euro-Communists and their glossy periodical, Marxism Today, had captured the party leadership while their opponents in the Morning Star fought for autonomy.

A third faction, the pro-Kremlin Straight Left, advocated closer ties with the Labour Party and, in particular, its left wing. Its initiator, Fergus Nicholson, had been a long-time recruiter of students and youth to the party.

He realised that there had to be some means of access to Labour, clearly the party of the working class, and resented “our forced exclusion” from the broad labour movement.

Straight Left was both a faction within the Communist Party and a non-aligned publication of the broad left.

Several of Mr Corbyn’s inner circle, including Andrew Murray and Seumas Milne, were associated from the outset with Straight Left. Their opponents in the Communist party accused them of “Labourphilia”. Mr Corbyn had no reservations about working with them and began to write regularly in the Morning Star.

They achieved their lifelong dream of controlling Labour when Mr Corbyn accidentally became party leader in 2015 — the barriers which had kept the far left outside for a century had been dramatically removed.

They maintained their affection for the Kremlin and simply transferred their allegiance from Leonid Brezhnev to Vladimir Putin — supportive of the Soviet invasion of Afghanistan in 1979 and quiescent in the annexation of the Crimea in 2014.

They remain reluctant to criticise Bashar al-Assad of Syria, a long time Soviet ally. Mr Corbyn was hesitant to point the finger at Russian military intelligence over the Skripal poisonings.

Many Corbynistas came of age during the period of decolonisation. They therefore identified in the 1960s with the nascent Palestinian national movement rather than with Israel.

Even though Stalin had promoted the establishment of a Hebrew republic in 1948, he refused to allow Soviet Jews to leave for Israel. Many were instead sentenced for Zionist activities to long terms in strict regime labour camps. It was therefore possible to accept Israel’s existence, but to oppose Zionism.

The Corbynistas could not overcome the fact that the Soviet Union had recognised Israel in 1948. Hence the continued support for a two-state solution while simultaneously opposing Zionism as “a racist endeavour”. They logically sought out peripheral anti-Zionist Jews rather than engaging with Jewish organisations which did not disavow Zionism.

It became possible therefore for the Corbynistas to support “a state of Israel”, but not one which is Zionist and one that does not necessarily have to have a Jewish majority. As Mr Milne has argued, any settlement between Israelis and Palestinians would require some “reversal of the historic ethnic cleansing”.

Mr Milne came of political age with the ascendency of the Likud in the late 1970s. His return from a left-wing Grand Tour of the Middle East left him committed to the Palestinian cause on his return to Oxford University. Like the PLO, he opposed the Begin-Sadat Camp David agreement of 1979.

The 1980s however proved difficult. While Thatcherism was embedding itself in the UK, Gorbachev was advocating glasnost and perestroika in the USSR — and even more alarmingly, Arafat was seeking a pathway towards rapprochement with Israel.

Many identified with the rejectionists in the PLO — those who declared the handshake between Rabin and Arafat on the White House lawn to be nothing more than a betrayal.

Mr Milne’s inability to recognise the raison d’être for the rise of Zionism was more recently encapsulated in his reported encouragement of Ken Livingstone to make his inaccurate “Hitler and Zionism” comment.

Whereas many Palestinian nationalists have moved to an accommodation with Israel, Palestinian Islamists have not. For Mr Milne, the central point is Palestinian resistance to Israel and not the political colouring of those who resist — even if reactionary and antisemitic. Mr Corbyn has happily followed Mr Milne in not discerning between advocates of the Palestinian cause.

Mr Milne warned the left in the past to “aggressively police the line between anti-Zionism and antisemitism”. Yet the reality of the last few years do not bear this out. It is the difference between theory and practice. It is also the lesson that British Jews have learned — the hard way — from history.

Mr Milne once commented that “for all its brutalities and failures, Communism... delivered rapid industrialisation, mass education, job security and huge advances in social and gender equality” — but it did not deliver for the Jews as a people.

There are many Jews who wish to repair this country and desire a just settlement with the Palestinians. The Corbynistas in the depths of their ideological blindness have gone out of their way to alienate too many Jewish progressives.

Like the Bourbon monarchs of revolutionary France, the Corbynistas have forgotten nothing and learned nothing from the lessons of history.

Colin Shindler is emeritus professor of Israel Studies at SOAS, University of London