As a connoisseur of British political scandals, I have long puzzled over one of the most fascinating affairs of its kind: did the Liberal Edwardian-era Prime Minister HH Asquith have sex with Venetia Stanley, a woman young enough to be his daughter? She certainly took up much of his time and attention in August 1914, when he should have been concentrating solely on the conflict that became World War I.

The same question now concerns bestselling thriller author Robert Harris, whose latest novel abyss – out later this month – focuses on those crucial summer days when Britain teetered on the brink of war before plunging into the abyss that arguably destroyed European civilisation.

Harris examines the crisis through the prism of Asquith’s relationship with the young Venetia, the noble daughter of a Liberal peer, who caught the priapic Prime Minister’s eye when he developed a crush on her during a Mediterranean cruise in 1912. They had met two years earlier, when the Prime Minister was 54 and Venetia just 19.

Harris’ book has not yet been published, but as far as I know, he believes – unlike more cautious historians – that Asquith did indeed have a physical affair with Venetia.

There is no doubt that the veteran prime minister – who was practically an alcoholic nicknamed “Squiffy” – was a womaniser, known for being “unsafe in taxis”. According to the spiteful literary critic Lytton Strachey, he had a habit of taking young ladies by the hand and leading them to his crotch.

After that momentous Mediterranean cruise, despite the 35-year age difference, Asquith bombarded Venetia with love letters – sometimes as many as four a day – full of political gossip and doggerel declaring his undying passion for her. His infatuation reached its peak in the fateful summer of 1914, when almost continuous Cabinet meetings debated whether Britain should take the plunge and join the burgeoning war between the major European powers.

Historians have reason to be grateful to Asquith for his unwise passion, for the letters he wrote to Venetia during these same meetings (when his Cabinet colleagues assumed he was taking notes on their views) are often our only source of information about his government’s important decisions on matters of war and peace.

Even when his brilliant eldest son Raymond went to the front, Asquith forwent a farewell meeting with him so that he could instead go to the London Hospital to see Venetia in her newly acquired nurse’s uniform. Raymond was later killed at the Somme.

Unsurprisingly, the love affair between the hapless Prime Minister and the cool young lady did not run smoothly. One problem was that another family member – his daughter Violet – had developed a lesbian crush on her friend Venetia and was also courting her with love letters.

Perhaps worse, another guest on that fateful 1912 cruise had also succumbed to Venetia’s charms. He was Edwin Montagu, a protégé of the Prime Minister, whom he had just promoted to minister. Unknown to Asquith, Montagu was beginning to pester Venetia with love letters and marriage proposals. Montagu’s amorous proposal was doubly problematic. First, he was widely regarded as not only dull but also repulsively ugly. With a shudder of disgust, Venetia spoke of his “oversized head and pockmarked face” and told a jealous Violet that she could not bear to kiss him.

Second, Montagu was a wealthy Jew, and his father had threatened to disinherit him if he married a woman outside his faith. Nevertheless, he continued to court Venetia relentlessly at the start of the war, and although she initially rejected his marriage proposal, she became disturbed by the Prime Minister’s increasingly frenzied courtship attempts. In 1915, in the midst of an existential government crisis, she informed Asquith that she would marry Montagu after all and convert to Judaism to secure his inheritance.

Although Asquith told Venetia that her news had “broken his heart”, he quickly recovered and even forgave Montagu, promoting him to the Cabinet. Indeed, true to his reputation, the Prime Minister was soon engaged in erotic correspondence with a new lover: none other than Venetia’s younger sister Sylvia. Asquith’s long-suffering second wife Margot – who had borne the Prime Minister five children – was accustomed to her husband’s extramarital tendencies and spoke tolerantly of the “little harem” he had assembled around him.

Sylvia was married to a front-line officer who (almost needless to say) also corresponded with Venetia. The younger sister was well aware of Asquith’s reputation as a groper and was careful to keep him at a safe distance when they were alone.

The Montagus’ marriage was not destined to be a success. Whether or not Venetia was a virgin bride when she walked down the aisle, she certainly had a fulfilling love life afterward. The only child of their marriage, Judy, later a close friend of Princess Margaret, was almost certainly not Montagu’s father, but one of her many lovers.

The unlikely couple acquired a magnificent Tudor mansion, Breccles Hall in Norfolk, and spent even more money from Montagu to commission the famous architect Edwin Lutyens to modernise the house and gardens. Winston Churchill, a close friend of the couple, was a frequent visitor and painted an enchanting view of the interior.

In 1916, the dynamic David Lloyd George, angered by his boss’s half-hearted conduct of the war, outwitted Asquith, overthrew him and formed a new coalition with the Tories. The Liberals split bitterly into the Asquith and Lloyd George factions and were never to form a single-party government again. Montagu betrayed Asquith and joined Lloyd George’s government as Secretary of State for India.

In 1924 he fell ill with a mysterious illness, died aged just 45 and was buried on the Breccles estate. Venetia became a very cheerful widow and had affairs with many men, including the press magnate Lord Beaverbrook and the immensely wealthy scientist, spy and polymath Victor Rothschild. In middle age she took up flying and courted the young pilot who flew with her around the world.

Shortly before his death in 1928, Asquith took a long car trip to Breccles for a final, sentimental meeting with Venetia, a regular guest of the Churchills during World War II. She died of cancer in August 1948, aged 60, and despite their differences, her ashes were buried next to Montagu in Breccles, leaving a mystery: had she had an affair with old “Squiffy” or not?

Does such old chatter matter? I would argue that it mattered a great deal, because when the government and the nation were deeply divided over whether or not to enter the war, the Prime Minister was distracted from his duties and more interested in sharing military secrets with his mistress than with his own Cabinet.

Asquith’s behaviour, whether he seduced Venetia or not, was stupid, reprehensible and irresponsible. In our #MeToo age of social media, it would be impossible to hide it, but in this respectful era, he got away with it.