https://www.ndtv.com/world-news/the-mole-in-mi6-how-a-cambridge-spy-fooled-british-intelligence-for-decades-8477386

Long before his career began, Philby had already pledged loyalty, not to Britain, but to the Soviet Union.

Philby was recruited in 1934 by a Soviet agent known as "Otto." It was his first wife, Lizzy, a committed communist, who connected him to the underground Soviet network. From that moment on, he became a double agent, sending British secrets to Moscow for nearly 30 years.

MI5 gave him the codename "PEACH." To the Russians, he was a goldmine.