the notorious Cambridge spy ring double agent

de bene esse: literally, of well-being, morally acceptable but subject to future validation or exception

British MI6 chiefs tried to protect notorious Cambridge spy ring double agent Kim Philby as they thought he 'couldn't possibly be a traitor'

Tim Milne, an MI6 spy who was Philby's oldest friend, has revealed the extraordinary lengths to which intelligence officials went in order to protect the notorious figure.