For almost half a century, Bertram Mills Circus was a household name throughout Britain among both children and adults and its Director, Cyril Bertram Mills, was one of the best-known and most influential names in the country's entertainment business.

But for forty years, Cyril Mills had also enjoyed a top-secret and wide-ranging career in British intelligence: obtaining the best aerial intelligence on Nazi rearmament for MI6 before the Second World War; becoming the first case officer to monitor the best double agent (Garbo) of the war after joining MI5; and working part-time during the Cold War 'for MI5 or 6 or both without being paid a penny'.

Remarkably, no word of Mills's secret career appeared in public until he was over eighty. Nobody suspected that the glamorous world of pre-war circus entertainment had been an extraordinarily fitting rehearsal for the lethal arena of deception and surveillance. In this remarkable true story, Christopher Andrew, best-selling official biographer of MI5, brings to life one of the most surprising and fascinating tales of espionage ever told.

Praise for The Defence of the Realm: The Authorized History of MI5:

'Sensationally good...A riveting story, the real-life spooks and spies far more compelling than anything you will see on the screen...history doesn't come more fascinating than this.' - Evening Standard

'Unprecedented...Weighty, measured and compelling...With this book, the author has done a formidably good job for both the service and the public interest.' - Max Hastings, Sunday Times

'As gripping as any thriller...Throws new light on an important area of the running of the country, analysing the changing threats to national security over the 100 years and discussing the appropriateness or otherwise of the service' s response.' - Stella Rimington, Financial Times