His lowly clerical grade did not mean he was denied access to innumerable secret documents. In Moscow, where he was posted aged 29 and entrapped with contemptuous ease by the KGB within months, he made an excellent impression on his superiors. Their reports commended the young man's "first- class appearance and manners", his unruffled comportment, readiness to please and exemplary moral standards.
In the wake of the notorious Foreign Office spies Burgess and MacLean, who had defected to Moscow in 1951, much was made of ever more rigorous vetting procedures designed to appease American fury over British security laxity. Homosexual behaviour was still a criminal offence in the United Kingdom and the Soviet Union. This meant that sexual entrapment along precisely the lines experienced by Vassall was such a danger that anyone vulnerable should have been denied sensitive access.
After Vassall's arrest in 1962 it became apparent yet again that the KGB were far more adept at spotting vulnerable individuals than were the Foreign Office personnel department. It also emerged that Vassall's selection for Moscow, of all places, had been in part an economy measure. Traditionally, his job had been performed by a married man, but to avoid allowances for couples they had posted a bachelor instead.
His treachery had been rewarded with plenty of cash after initial threats to send his mother photographs of her son enjoying a homosexual orgy. Vassall spent lavishly on clothes and frequent holidays at a time when only the rich could afford to follow the sun. The rent of his Dolphin Square flat alone was not far off his entire income after tax.
These facts proved damaging to the Macmillan government, which was already under pressure after another naval specialist, George Blake, had been sentenced to a record 42-year sentence for spying. Vassall's own trial by Lord Parker, the Chief Justice, was almost entirely in camera, yet the press, despite ferocious denials from official sources, uncovered a hapless saga of incompetence, extravagance combined with foolish penny- pinching, and sexual corruption. After Vassall, and even more when the Profumo-Keeler scandal broke in 1963, Macmillan's premiership was dogged by a sleaze factor.
These political reverberations lent significance to an intrinsically sad story. Born in St Bartholomew's Hospital in London, where his father was a long-serving chaplain, Vassall developed a boyhood taste for religious pageantry in the twin churches of St Bartholomew in West Smithfield. At school in Monmouth he discovered his homosexuality and was disappointed in an ambition to enter Keble College, Oxford.
Instead, he joined the RAF in the ranks, where ironically he received the photographic training that made him so competent a spy. A state-of- the art Praktina document-copying camera was found expertly concealed in 807 Hood House, his Dolphin Square address, when section DI of the security services stripped the place.
After wartime service with the RAF, in 1948 Vassall joined the Admiralty. While in Moscow he had printed a special card reading "Junior Military Attache", and had even been rebuked for turning up at social occasions thought to be too elevated for his grade. Back in London (from 1957), he used his spymaster's cash to cut something of a swathe in the underground world of homosexuality. His sartorial role model was the Hon Thomas Galbraith, the junior Admiralty minister he served as personal secretary before moving to military intelligence. He kept a silver-framed photograph of his boss in naval uniform on his desk.
In retirement, Vassall's father had become an ex officio curate at St James's, Piccadilly and his son made some play of his connection with this then fashionable establishment.
He liked to impress friends with connections in high places and often cited Lord Foppington, a character in Vanbrugh's play The Relapse, to the effect that this was the sole church in London with a congregation compiled entirely of gentlemen. He was also wont to repeat compliments he said he had received for his "bedroom-eyes".
The importance of his espionage disclosures were never revealed. The tribunal set up under Lord Radcliffe established that there had been no impropriety in his relationship with Galbraith who, though he felt obliged to resign, later received a more senior government job. The main victims of Radcliffe were the press, two of whom served jail sentences for refusing to name sources.
Having converted to Catholicism, Vassall proved a model and increasingly religious prisoner, whose spiritual life was enriched by visits from Lord Longford. Released after serving ten years, he claimed in his autobiography that he was "a pygmy of a spy" in comparison with the atom physicist Klaus Fuchs. Nonetheless, Fuchs's sentence (14 years) had been four years shorter than his own.
Vassall was certainly the smallest of beer compared with the Cambridge Five: Burgess, MacLean, Philby, Blunt and Cairncross. Unlike them he had scant ideological regard for Communism. He had operated entirely under threat of blackmail and also for greed.
Victim of historical circumstance as much as anything, he might in another age have found a vocation as a gay cleric. As it was he changed his name to John Phillips and spent his declining years in total anonymity and obscurity in St John's Wood, north London.
William John Vassall, spy: born London 20 September 1924; died London 18 November 1996.