J Edgar Hoover, director of the FBI, was one of the most powerful and omniscient figures in the United States for almost 50 years.

However, when he wanted to send a letter of thanks to Sir David Petrie, the Director-General of MI5, in 1943, he had no idea what address to put on the envelope.

He wrote to Sir David to thank him for welcoming and assisting the first FBI liaison officer sent to London to co-operate with MI5 in anti-espionage operations. It was a breakthrough in co-operation between the two agencies and helped to cement the special intelligence relationship that exists to this day.

Hoover signed the letter, but there was clearly some doubt as to where to send it. In the end, the address selected was: “British Security Service, London, England.”

The envelope, covered in different scribbled suggestions by various post offices as to where it should be sent, was regarded as so unusual that MI5 kept it and put it in a secret file marked “Liaison with the FBI” that was declassified yesterday and released by the National Archives.

The letter from Hoover was sent from one address to another. A note to Sir David in the file said: “You may wish to have your attention drawn to the fact that owing to the curious way in which this letter to you from the FBI has been addressed, the Post Office sent it to Leconfield House, W1 (perhaps because there is a Security Office there), then to USA Army HQ, and finally to the War Office.”

The note added that, at the War Office, someone with intuition passed it on with a question mark to Room 055, the proper postal address for MI5. “This letter has thus been wandering about London for three days before reaching you,” Sir David was told.

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