The allegations of antisemitism currently swirling around the Labour Party are shocking but also unsurprising.

Labour has a long and honourable history of fighting racism in all its forms. Since the party's birth, many Jews have been proud members, supporters and leading lights. From Harold Wilson to Gordon Brown, Labour has been led by men whose commitment to support the state of Israel has been unshakeable.

But there is another, darker side to the party's history and to that of the wider left. For whenever and wherever the hard left has gained a foothold, it has invariably unleashed the virus of anti-Zionism.

It first infected the Young Liberals - whose chair, Louis Eaks, proclaimed that "Jews see themselves as the master race" and accused Israel of "brutal atrocities against humanity" - in the late 1960s.

The Liberal leader, Jeremy Thorpe, set up a special commission to investigate the behaviour of Eaks and his allies, dubbed the 'Red Guard' by the press. It found that the Young Liberals' attacks on Israel - "in terms totally at variance with the policy of the party" - were calculated to upset Jewish voters and had done so.

The hard left tide had crested. Labour's moderates fought back.

Thorpe publicly apologised but appeared powerless or unwilling -in 1974, he welcomed the defection to the Liberal benches of the virulently anti-Israel former Labour minister, Christopher Mayhew - to halt what Eaks had unleashed.

Unsurprisingly, the virus soon spread to Britain's campuses. When the UN General Assembly passed its infamous resolution equating Zionism with racism in 1975, the hard left seized its chance to apply the National Union of Students' policy of denying a platform to racists and fascists, aimed originally at the National Front, to Zionist speakers and Jewish student groups which supported Israel.

In just two terms, more than 20 universities and colleges debated, and 11 passed, resolutions calling for the destruction of Israel, labelling Zionism as racism and supporting the PLO.

The atmosphere on some campuses became toxic for many Jewish students. At York, the student union attempted to strike the Jewish Society from its register; at Salford, a 43-point 'indictment' declared that "the South African fascists, the Zionists and fascists' shared a common belief in 'race superiority"; and at the University College of Wales, Swansea, the student newspaper warned of the "Zionists' plot to conquer the world".

The unholy alliance between hard left and extreme right was evident when the National Front began distributing antisemitic leaflets praising the York and Salford student unions. The NUS leadership eventually took action. But by now anti-Zionism had already begun to seep into the wider body politic.

While Wilson and his successor Jim Callaghan held Labour's reins, the party remained largely immune from it. But after its defeat in 1979, the hard left burst forth, using the power it had stealthily amassed at the grassroots level to attempt to seize control of the party nationally.

This was evident just one month after Mrs Thatcher became Prime Minister, when the Labour Party in Hackney North and Stoke Newington passed a motion declaring its opposition to the 'Zionist State of Israel', and endorsing the "struggle of the Palestinian people for the liberation of their homeland". The resolution provoked anger in Hackney's Jewish community and was bitterly attacked by Stanley Clinton-Davis, the MP for neighbouring Hackney Central and a former minister. By 1981, attempts to oust the Jewish MP and replace him with a hard left Hackney councillor were under way.

The assault in Hackney was simply the opening salvo in a ferocious anti-Zionist campaign within the party. In Brent, for instance, the former leader of the council, John Lebor, a member of the National Executive of Labour Friends of Israel, was driven to advise residents to vote SDP.

Israel's invasion of Lebanon in 1982 raised the temperature still further.At a meeting of the Paddington party, loud applause greeted the remarks of a local councillor who argued that Israel - a "racist, theocratic state" - had "no right to exist" and had come about because of the world's "guilty conscience" about the Holocaust.

In a display symptomatic of the party's chaotic state on the eve of the 1983 general election, Labour's annual conference in 1982 passed a series of apparently contradictory resolutions on Israel. Most of the attention, however, was focused on the adoption of one proposed by the hard left leader of Lambeth council, Ted Knight, which, by endorsing "a democratic secular State of Palestine", was viewed as challenging Israel's right to exist.

After its calamitous defeat in 1983, the party's new deputy leader, Roy Hattersley, acknowledged the "creeping antisemitism operating in the Labour Party", labelling it "absolutely intolerable". Under Neil Kinnock, warm words were matched by action to rid Labour of hard left entryists, and thus quieten anti-Zionist rhetoric and adopt a more balanced position on the Israeli-Palestinian conflict.

The tide was only slowly turned. Two years after Hattersley's pledge, Labour's general secretary, Larry Whitty, was forced to condemn local moves to disaffiliate Poale Zion, the Jewish socialist society (precursor to the Jewish Labour Movement) and admit that "in some areas of the party, any apparent support for Israel gets attacked and there is a dangerous spillover from legitimate criticism into anti-Zionism, which in some cases comes pretty close to antisemitism".

Moreover, one voice Kinnock couldn't quieten was that of Ken Livingstone, the leader of the GLC. The charge-sheet against him is well-known. The bizarre interviews, like that in 1984 when he warned that the Jews "have been organising here in London, and throughout Britain, into paramilitary groups which resemble fascist organisations". The anger provoked by the publication of a cartoon of Menachem Begin dressed in a Gestapo uniform amidst the skulls and corpses of Palestinians with the caption 'The Final Solution' in a newspaper of which Livingstone was an editor.

The manner in which even worthwhile causes, such as the GLC's 'Anti-Racist Year', swiftly descended into a platform for the hard left to attack Israel and justify terrorism against it.

The abolition of the GLC failed to remove Livingstone's long shadow from the London Labour party. Instead, it now fell on Brent East, where he unseated the sitting MP, Reg Freeson. The grandson of Jewish immigrants who had fled the Russian pogroms of the 1890s, Freeson was, like Ian Mikardo, on the party's left and a staunch Zionist. "It was a lot of other things," he said afterwards of his deselection, "but there was a bit of "Jew-boy" mixed in it."

In 1987, Livingstone managed to hold the seat, although Freeson's 1983 majority was slashed. After the election, the number of Jewish Labour MPs fell to its lowest level since 1935.

But the hard left tide had crested. Labour's moderates fought back. A new generation of Jewish Labour activists who would serve the party in parliament in the 1990s and beyond - Louise Ellman, Ivan Lewis, Margaret Hodge, Gillian Merron, Fabian Hamilton and Barbara Roche - were serving their political apprenticeships in local government and elsewhere. The virus of anti-Zionism had been destroyed.

Under Ed Miliband, however, the warning signs of its re-emergence were ignored and the hard left went unchallenged.

The past is prologue, and so it proved.