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 on 4 April. 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."

The shadowy world and double-dealing of Philby and others in the Cambridge Five spy ring – such as Anthony Blunt, Guy Burgess and Donald Maclean – has long fascinated audiences. In one excerpt from the Stasi lecture, Philby detailed how he was ordered to undermine his boss so he could take over as head of an MI6 section responsible for unmasking Soviet agents, calling it a "very, very dirty story".

"After all, our work does imply getting dirty hands from time to time," he said. "So I set about the business of removing my own chief. You oughtn't listen to this," he told the East German spies, provoking laughter.

Philby, stationed in Washington as the liaison officer between the CIA and MI6 from 1949-51, said he escaped detection for so long because he was part of the British governing class system. Many MI6 colleagues had much to lose as they had been involved in his recruitment and promotion, he said.

He fled to Moscow in 1963 when new evidence of his work for the Soviet Union arose. He alleged he was able to escape from Beirut because the MI6 agent sent to watch him was an avid skier and had gone off to the Lebanese mountains after hearing news of a snowfall.

Philby's hour-long address ends with simple advice to the spies: "Deny everything. My advice to you is to tell all your agents that they are never to confess."