[2] Historian Keith Jeffery was commissioned to write a similar authorised history of the Secret Intelligence Service (MI6) for release in 2010.

The Daily Telegraph called it "magisterial, authoritative, balanced, readable ... full of wry humour and with an eye for the absurd",[4] declaring that "MI5 could not have wanted a better historian than him.

[5] More tempered praise came from The Sunday Times, where Max Hastings found the account "weighty, measured and compelling", regretting only a bit of occasional overenthusiasm on the part of the author and a lack of reflection on "the relationship between the service, ministers and the public".

[6] Ben Macintyre reviewed Defend the Realm for The New York Times Book Review, calling it "not only a work of meticulous scholarship but also a series of riveting and true spy stories", though recognising that such an account on a secretive organisation will necessarily be incomplete in certain areas.

[8] The book was criticised in Quadrant magazine in an article by Paul Monk for almost non-existent use of meaningful citation, bias in favour of MI5's "official" line, and for glossing over the issue of whether Roger Hollis was a Soviet agent.