Complex sentence: David Cornwell (John le Carré), who worked as an intelligence officer for both MI5 and the SIS (MI6), has said that Philby betrayed his identity to the Russians, which was a factor in the 1964 termination of his intelligence career.
Split into simpler sentences: David Cornwell (John le Carré) worked as an intelligence officer for both MI5 and the SIS (MI6). He said that Philby betrayed his identity to the Russians, which was a factor in the 1964 termination of his intelligence career.