In a letter to The Observer, Howard Jacobson, Simon Sebag Montefiore, Sir Simon Schama, Neil Blair, Tracy-Ann Oberman, and Rabbi Julia Neuberger, write:

Over the past four years, we have watched the unchecked spread of vicious antisemitism – anti-Jewish racism – in the Labour party with bewilderment and disgust – all the more so since we respected Labour’s long previous history of fighting racism, and its many brave and decent members.

Now, after the BBC’s Panorama programme, we can see for ourselves what is and isn’t false. The extent of Labour’s antisemitism, the degree to which it has been protected, sanctioned and propagated by the leadership faction, can brook no further denial. It is clear this is not just about Jeremy Corbyn but the pernicious world view of a faction that has – temporarily, we hope – taken control of Labour.

But yet again the Labour leadership remains determined to go on piously protesting its innocence, rejecting the message and blaming the messenger.

We are where we were before. Whistleblowers would be heroes to any other Labour party; to this Labour party they are traitors.

In our view, those whistleblowers and Labour members who could no longer stomach what was happening in a party they loved, and were in some cases driven to the point of nervous breakdown by it, have shown a rare moral courage and decency that belongs in a great tradition, and gives us hope. In the meantime, trust between Corbyn’s Labour and most Jews in Britain appears to be fractured beyond repair. The very fact that Labour is, like the BNP before it, being statutorily investigated by the Equality and Human Rights Commission for racism is not a matter of housekeeping, but a taint of international, historic shame.

There are procedures that could take the heat out of this – a fully independent complaints body, for example, over which Corbyn and his inner circle exert no covert influence. Not with the aim of shuffling off blame, but in order, finally, that the party will honestly own up to what can no longer be filed away in a folder. After Panorama, damage limitation should not be the first priority for Labour. Draining the antisemitism that lies like a pool of poison in the party’s soul is. Now, nothing else will do.

A few years ago, there was some legitimacy to the claim that some supporters of Israel were cynically engaged in hasbara when they conflated criticism of Israel with antisemitism.

In 2009, the Israeli filmmaker, Yoav Shamir, released his documentary, Defamation, in which he exposed the efforts of organizations such as the Anti-Defamation League in the U.S., to amplify concerns over antisemitism where evidence of its actual extent was commonly being overstated.

Ten years later, if Shamir was to revisit the same issue, he would be forced to tell a very different story.

What changed?

There are multiple strands to the growth of anti-Jewish hatred, but the one that afflicts the Labour Party can surely be traced above all to the popularization of the so-called pro-Palestinian movement.

I say “so-called” because what was long a political movement supporting the self-determination of a population living under military occupation, has in recent years metastasized by becoming, at least for some of its non-Palestinian supporters, less pro-Palestinian and more anti-Israeli.

In 2011, President Obama’s ambassador to Belgium, Howard Gutman, was harshly criticized for stating the obvious: that Israel’s military actions targeting Palestinians were fueling the rise of antisemitism.

Gutman said that “throughout the Muslim communities that I visit, and indeed throughout Europe, there is significant anger and resentment and, yes, perhaps sometimes hatred and indeed sometimes an all too growing intimidation and violence directed at Jews generally as a result of the continuing tensions between Israel and the Palestinian territories and other Arab neighbors in the Middle East…every new settlement announced in Israel, every rocket shot over a border or suicide bomber on a bus, and every retaliatory military strike exacerbates the problem and provides a setback here in Europe for those fighting hatred and bigotry here in Europe.”

Hostility towards Israel rose dramatically during the 2008-09 war on Gaza and was further reinforced by a widely condemned economic siege that continues to this day.

But along the way, it became clear that for many people concerned with this issue, their sense of injustice and outrage increasingly expressed itself more through antipathy directed at the perpetrators of this injustice rather than as solidarity with its victims.

When Palestinians in Yarmouk outside Damascus suffered just as egregiously as those in Gaza, the pro-Palestinian movement largely remained silent.

The equivocations commonplace among supporters of Israel who are reluctant to criticize the actions of its government, were echoed by antiwar and anti-imperialist activists who were reluctant to condemn the actions of Assad and his Russian and Iranian allies — lest such criticism might imply support for a long-predicted Western intervention in the war in Syria.

Where there was no shortage of self-ascribed moral clarity in viewing the conflict between Israelis and Palestinians, the conflict between a brutal dictatorship and its opponents was supposedly far too complex for Middle East watchers to reliably decipher.

As Elizabeth Tsurkov wrote last year:

For the past two weeks, horrifying images of Palestinians being murdered have surfaced on social media. Jets have dropped bombs, sending flames and black smoke into the air, followed by helicopters dropping improvised and unguided barrel bombs. On the ground, in the besieged Palestinian camp, buildings collapsed burying civilians alive underneath them.

Those who have attempted to flee the camp have been arrested. No food or medicine has been allowed into the camp since July 2013. About 200 residents have starved to death or died due to lack of medicine. The airstrikes and shelling have not stopped for two weeks, preventing rescuers from retrieving the rotting corpses.

And yet, despite these horrors, no significant protests took place anywhere in the West or Arab world. The traditional champions of the Palestinian cause — those same people who have protested Israeli attacks on Gaza — have remained silent as the Palestinian Yarmouk refugee camp near Damascus in Syria was shelled by Russian and Syrian air forces, while militias loyal to the Assad regime enforced a brutal siege.

The silence surrounding the brutalization of an entire Palestinian population exposes something few have spoken about: that pro-Palestinian sentiment is often just anti-Israel or anti-American sentiment dressed up in disguise. And when it comes to Syria’s starving, dying Palestinian population, the pro-Palestine left is nowhere to be seen.

There is a larger theme here with echoes across the political spectrum. It involves the exploitation of the easiest route for galvanizing political unity: that is, rather than trying to articulate and advance political goals, it’s much easier to identify and vilify a common enemy. Everyone can instantly stand together when they agree about who they stand against.

It’s a ploy that works just as well for protest leaders or presidents, populists or dictators.

Politics based on opposition, whatever its radical trappings, has a tendency of reinforcing the status quo. It’s all about taking a self-righteous stance, rather than moving forward and taking the risk of stumbling as you go.

But the politics of the common enemy all too easily creates common ground with the likes of Saddam Hussein, Muammar Gaddafi, Bashar al-Assad, and Vladimir Putin.

For Jeremy Corbyn and his close advisers, their habitual political reflex is to regard every challenge as an enemy assault.

What they are finding too hard if not impossible, is to acknowledge their own role in enabling the growth of a malignancy that is now destroying their own party.