London, Dec 28 : The Soviet Union had used civil airliners to conduct secret Cold War spying missions over Britain, according to newly published government files.

Papers released by the National Archives under the 30-year rule revealed that some aircraft would switch off their transponders, alerting air traffic controllers of their position before veering off their approved flight paths to carry out aerial intelligence-gathering missions over sensitive targets, express. co. uk reports.

In a memorandum marked SECRET UK US EYES ONLY, Defence Secretary John Nott informed former Prime Minister Margaret Thatcher in December 1981 that the RAF was monitoring the hundreds of monthly flights through UK airspace by Warsaw Pact airliners.

He wrote that 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, the report said.

According to the report, but that was not the only example of bad behaviour by enemy spies.

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 report added.

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, the report added. (ANI)