Cocktails, tennis, film evenings and skiing filled the lavish social diary of one of the KGB’s most notorious British spies, newly-released security service files reveal.

John Vassall, who was blackmailed into passing vital defence secrets to the Kremlin, was at the centre of one of the century’s most bizarre and sensational cloak-and-dagger spy scandals.

Newly-digitised MI5 files, published by the National Archives, also contain pictures of Vassall, as well as the cupboard housing a secret compartment in which films containing information copied from Admiralty files were found when police raided his London flat in 1962.

(This) lamentable case of espionage might have been avoided

The Admiralty was the government department that managed naval affairs at that time.

A 21-page diary of engagements at the British Embassy in Moscow, covering the period 1954 to 1956, gave an insight into the busy social life Vassall enjoyed.

Dinners and drinks events featured heavily, as did bridge, while there were also boat trips, theatre outings, art class, Scottish reels dancing, skiing, films in the garden and picnics in the country.

Mass was also noted fairly regularly in the diary.

Vassall, who was jailed in 1962, changed his name to John Phillips on his release a decade on and tried to live a quiet existence, turning to religion.

His funeral at the Brompton Oratory in Knightsbridge in 1996 was a Latin mass.

His treachery began when he was a junior Naval attache to the British Embassy in Moscow in the mid-1950s and was blackmailed into spying for the Russians after being set up and photographed with a gay partner.

After he returned from Moscow, he continued spying while he worked for Thomas Galbraith, a junior minister at the Admiralty as an assistant private secretary.

When suspicions were finally raised that Vassall was living beyond his means, his flat was raided and incriminating documents were found.

Following a high-profile trial, he was sent to jail and Galbraith, then MP for Glasgow Hillhead, resigned.

In the newly-digitised MI5 files, it is noted on one document that had reported suspicions about Vassall’s sexuality been considered back in 1955, the “lamentable case of espionage might have been avoided”.

Shortly after Vassall’s death, it was disclosed that the Admiralty had failed to act on a report warning about his conduct, which might have seen him sent home before being blackmailed into selling highly-classified defence secrets to the Kremlin for a period of seven years.