As a Cambridge lecturer, he recruited for the Soviet secret service and, as a member of the British Secret Service 1940-45, passed information to the USSR. In 1951 he assisted the defection to the USSR of the British agents Guy Burgess and Donald Maclean (1913-1983). He was the author of many respected works on French and Italian art. Unmasked 1964, he was given immunity after his confession. He was director of the Courtauld Institute of Art 1947-74 and Surveyor of the Queen's Pictures 1945-1972. He was stripped of his knighthood 1979 when the affair became public. KCVO 1956.
