Publisher Penguin has revealed some of the launch plans of what it calls an authorised history of Britain's internal security and counter-intelligence service, the famed MI5. It has described the access granted to its writer, Prof Christopher Andrew, as being ''unprecedented.''

The authorized history has been written by noted writer on intelligence affairs, and expert, the Cambridge historian, Professor Christopher Andrew. His effort on MI5, ''Defence of the Realm'' (Allen Lane, £30) will be published on Monday, 5 October 2009 with a press conference. Copies of the book will be made available at bookshops on the same day.

The initials MI stand for Military Intelligence and the numerical 5 distinguishes it from its sister service, MI6, which is charged with intelligence gathering and general hanky-panky overseas.

The book on MI5 borrows its title from the service's motto ''Regnum Defende,'' which translates as "Defend the Realm."

Two books have been commissioned on both the services for release in their centennial year as the services were founded in 1909. 

In MI5's case it came into existence in October 1909 as the "Home Section of the Secret Service Bureau". It has undergone a change in nomenclature on a number of occasions since, and has been referred to as the ''Security Service'' since 1931.

Revealing some details Penguin Press publishing director, Stuart Proffitt, said: "Absolutely no major intelligence organisation in the world has ever let an independent historian into its archives in this way. The book contains not only some very major news stories, but allows us to see for the first time in the round the role of this previously extremely secretive organisation in the history of Britain in the past century-through two world wars, the Cold War, and now the war against radical fundamentalist terrorists."

Proffitt also dropped hints that the book would reveal identities of "previously unknown enemies of the UK whose activities have been uncovered by the Service".

Of course, the book "inevitably gets thinner" when it comes very close to the present time, Proffitt said, but added: "It has been possible to say a good deal up to 1990 and after that as far as national security considerations allow. Chris has been allowed to see every file he has requested."

Christopher Andrew is Professor of Modern and Contemporary History, former Chair of the History Faculty at Cambridge University, official historian of the Security Service (MI5), Honorary Air Commodore of 7006 Squadron (Intelligence) in the Royal Auxiliary Air Force, Chair of the British Intelligence Study Group, and former visiting professor at Harvard, Toronto and Canberra.

His most recent book, ''The Mitrokhin Archive: The KGB in Europe and the West ''(Penguin), was based on the extraordinary collection of top-secret material from Russian intelligence archives smuggled to Britain by a former KGB archivist, Vasili Mitrokhin, with the help of MI6.

Christopher Andrew's other books on secret intelligence include ''For The President's Eyes Only: Secret Intelligence and the American Presidency'' and ''Secret Service: The Making of the British Intelligence Community.''

According to Proffitt, the service's history has been written and edited on MI5 premises, with the content not permitted to leave the building. "I was going in there once or twice a week to read and edit with Chris," said Proffitt.

The Penguin publishing director also described ''Defence of the Realm'' as "certainly one of the two or three most interesting" books he had worked on in his entire career.

An authorised history of MI6 by Professor Keith Jeffery is set to follow from Bloomsbury next year. Bloomsbury Publishing plc is an independent, London-based publishing house known for literary novels.

It is also the publisher of the immensely popular Harry Potter series by J K Rowling.