The Soviet Union used civil airliners to conduct secret Cold War spying missions over Britain, according to newly published Government files.
Some aircraft would switch off their transponders, alerting air traffic controllers to their position before veering off their approved flight paths to carry out aerial intelligence-gathering missions over sensitive targets, papers released by the National Archives under the 30-year rule show.
In a memorandum marked SECRET UK US EYES ONLY, Defence Secretary John Nott informed prime minister Margaret Thatcher in December 1981 that the RAF was monitoring the hundreds of monthly flights through UK airspace by Warsaw Pact airliners.
"One incident of particular interest took place on 9th November, when an Aeroflot IL62 made an unauthorized and unannounced descent from 35,000 ft to 10,000 ft just below cloud level, to fly over RAF Boulmer, a radar station currently being modernised. It subsequently climbed back to 37,000 ft," he wrote.
"During this manoeuvre its Secondary Surveillance Radar which automatically broadcasts the aircraft's height was switched off, though it was on before and after the incident. It must, therefore, be assumed that it was switched off intentionally to conceal a deliberate and premeditated manoeuvre.
"Our investigations have now revealed it was the same aircraft which over flew the USN base at Groton when the first Trident submarine was being launched. You will recall that as a result of this incident the President banned Aeroflot flights over the USA for a short period."
But that was not the only example of bad behaviour by enemy spies that year. In August 1981 the Second Secretary at the USSR embassy VN Lazin became the first Soviet diplomat for a decade to be expelled for "activities incompatible with his status".
The Foreign Office informed No 10 that Lazin, actually the senior member of the scientific and technical intelligence section of the KGB in London, was arrested during a "clandestine meeting" with a Portuguese national.
"He developed his relationship with the Portuguese national over several months and sought to obtain technical and scientific information in the UK from him and to use him as an agent with the possibility of eventually placing him in a Nato post," the Foreign Office noted.
The Soviets responded in traditional fashion with the tit-for-tat expulsion of the British cultural attache in the Moscow embassy. More was to follow six months later in February 1982 when MI5 decided to call time on the espionage career of another Soviet, Vadim Fedorovich Zadneprovskiy, a member of the Soviet trade delegation whom for the previous five years operated as a KGB agent-runner. His recruits included a British businessman who was given the codename COURT USHER.