London, April 4 (Reuters): British double agent Kim Philby detailed his life of betrayal and the ease with which he was able to pass secrets to his Soviet controllers in newly-discovered video footage broadcast by the BBC today.

Philby was one of the Soviet Union's most successful spies who penetrated the heart of the British establishment and passed secrets back to Moscow for three decades, part of a ring of British double agents recruited in the 1930s.

The video, discovered by the BBC in the official archives of the Stasi, the former East German Intelligence Service, shows Philby, who defected to Moscow in 1963 and died there in 1988, lecturing Soviet spies in 1981 about his life as an agent.

"Dear Comrades," he begins, delivering his lecture to a rapt audience in his upper class English accent.

Philby said he became interested in communism while at Cambridge University, and explained how he was recruited by Moscow in the 1930s to infiltrate Britain's Secret Intelligence Service, the foreign spy agency now known as MI6, the BBC said.

He disclosed how easy it was to steal secrets as he began to rise through MI6's ranks.

"Every evening I left the office with a big briefcase full of reports that I had written myself, full of files and actual documents from the archive," Philby said.

"I used to hand them to my Soviet contact in the evening. The next morning I would get the files back, the contents having been photographed and early the next morning I would put them back in their place. That I did regularly year in year out."