“SELF-HATING Jew” is a concept misused in recent decades to slur Jews critical of Israel’s deeds or the entire zionist project. But here it is entirely apposite.

Francis Beckett grew up knowing that his father John Beckett had been a left-wing Labour MP who threw in his lot with Oswald Mosley’s British Union of Fascists (BUF)before setting up the National Socialist League with William Joyce.

While Joyce, under his Lord Haw-Haw guise, broadcast nazi propaganda from Berlin during WWII — he was hanged at its denouement as a traitor — Beckett was interned in Holloway jail alongside Mosley.

Throughout this time and beyond the grave Beckett kept his secret, even from family, that he was Jewish.

His mother, whom he adored, was known as Eva Salmon but she entered this world as Eva Solomon, born of Jewish parents Mark Solomon and Jessy Isaacs.

Beckett, whose violently furious response to occasional rumours about his provenance served to convince doubters, claimed to emanate from generations of Cheshire farmers — his so-called yeoman ancestors.

The author suggests that his father delayed joining the BUF until his mother died, preferring not to upset her while she was alive.

This is an example of his tendency to sometimes place the best interpretation on Beckett’s behaviour, even while meticulously recording the case against.

He acknowledges that his father — to whom he refers throughout as John, undermining the objectivity required in biography — was a racist and an anti-semite.

But, he says, he was not a “bad” man. “In many ways, he was a rather attractive character, as the late Fenner Brockway, among others, confirmed to me.”

Brockway worked alongside Beckett in the Independent Labour Party during the latter’s pre-fascism and after he had become the youngest Labour MP, winning Gateshead in 1924 at the age of 30. He had built a reputation for hard-hitting socialist oratory and journalism and had already found friends in Labour’s higher echelons by then, lodging with and working for Clement Attlee in Limehouse.

He came into wealth, harvesting his friendship with actor-manager Arthur Bourchier to secure lucrative investments in West End and touring theatre companies and to send his daughter to private school while railing against the influence of the rich and powerful to run society in their own interests.

Beckett was for a time a wary ally of the Communist Party, referencing the hardship finance that found its way to his constituents in the Durham coalfield after the 1926 general strike was defeated.

However, he retained an antipathy to the party — shared by his biographer son — based on its links to Moscow and its influence on what he called “political Bloomsbury.”

The book’s subtitle can refer to the degeneration of a bright, impressive and popular socialist MP into a squalid anti-semitic hatemonger or the tribulation of a man living in lifelong denial about his true identity.

It may also allude to his son’s torment, as he understands and rejects the vile reality of his father’s obsession while holding firmly to the filial love for what he sees as the man behind the mask.

The sheer scale of the research and personal interviews he has carried out suggests a momentous cathartic process and the subject matter belies the pleasure of reading it.

JOHN HAYLETT