For 30 years Stephen De Mowbray has maintained a self-imposed silence on a career that once took him to the heart of one of British intelligence's most controversial episodes.

In 1979 he quit his job with the Secret Service because he believed officials had failed to take seriously the claim that British intelligence had been further penetrated by its enemy - the Soviet Union's KGB.

A number of spies had been discovered in the 1960s but De Mowbray believed there were more. But he found no-one at the top willing to listen.

Three decades later, De Mowbray decided to tell his side of the story after reading the authorised history of the Security Service (MI5), published last October.

It dismisses the view that there were further traitors in the Security Service.