Britain’s ambassador to Moscow in the 1930s confessed in a secret telegram to the Foreign and Commonwealth Office in London that he had broken the strict rules and had bought “an enormous amount of black-market roubles”.

Sir Esmond Ovey had previously resisted the temptation, although all his fellow diplomats in the other missions in Moscow regularly went to Riga where the rouble exchanges took place.

He received a ticking off from the Foreign Office and was told that, if he got found out, he would be on his own. He would receive “no official support from the FCO”, an MI5 report released by the National Archives revealed.

What neither the Foreign Office nor Sir Esmond knew was that the Russians had broken the British cipher codes and were able to read everything going back and forth between London and Moscow. Even Joseph Stalin, the Soviet leader, knew about Sir Esmond and he quickly took action to embarrass the ambassador.

The revelation emerged many years later when a Soviet hitman called Leon Helfland - who had a record of “particularly brutal murders” - defected to the United States in 1940 and a British official was allowed to question him.

The declassified MI5 file on Helfland recorded his claim that the Russians read all the British telegrams, and a further allegation that a spy was working for Moscow inside the British embassy in Rome. Although this was an eye-opener for the Foreign Office, one senior British official knew all about Moscow’s espionage successes at that time - Harold “Kim” Philby, an MI6 officer and long-time double agent for the KGB. His name cropped up in the Helfland file as someone with extensive knowledge of, and involvement in, the case. Philby was not suspected as a traitor until the 1950s.

Under questioning in New York in 1941, Helfland told the British official that the telegrams relating to the black-market roubles “were shown to Stalin, who promptly closed the only shop where diplomats could purchase goods, leading Ovey with a large amount of roubles which he could not spend”.

Sir Esmond complained about the closure of the shop to the Soviet commissar for foreign affairs. But the Russian knew about Sir Esmond’s rouble-dealing and “spent a happy hour informing Ovey (while Ovey might not credit it), certain diplomats had been buying roubles on the black market”.

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