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Prof Christopher Andrew, a historian and friend of Mitrokhin who has written two books on the archive, said caches were hidden around most major cities.

Some of the caches hidden around Europe have since been uncovered and, although the archive provides no details of hidden weapons in Britain, many are almost certainly here, he said.

He added: “Given that Britain was second only to the United States in terms of importance to the Soviets at this time, it would be remarkable if this tactic wasn’t deployed here. Of course by now they would not be easy to find and it is unlikely the weapons would be serviceable.” Describing one stash near Berne, Switzerland, Mitrokhin, who died in 2004, provides directions to a chapel near a farm: “After taking 36 steps, you will be at the point between two large leafy trees, the only ones in the sector.

“The distance between the trees is three paces. The area between the trees has been used for the cache.”

Another note provides instructions on how to disarm explosive booby-traps on the caches.

Mitrokhin defected in 1992 after the fall of the Soviet Union, bringing more than 25,000 pages of secret files to the British embassy. “Once there, he opened his suitcase, revealing the documents along with his dirty underpants and food he had packed for the journey and asked to speak to somebody in authority,” Prof Andrew said. Once the value of his offer became clear, he and his family were brought to Britain.