Officially, you’ll find MI5 in London’s Thames House, the most imposing and well-secured building on Millbank. There are rumours, of course, that the footprint of Britain’s domestic intelligence agency spreads far wider, into shabby, anonymous premises of the kind seen on the Apple TV series Slow Horses and even through secret tunnels under the river.
For much of its history, the organisation wasn’t even a matter of public record: until the 1989 Secret Service Act gave it statutory recognition, ministers routinely denied all knowledge of its existence. It is world famous, yet nobody on the outside has the faintest idea what its day-to-day in 2026 actually looks like — those who claim to know, and are willing to share that information, by definition do not.
Its historic paper trail, however, is a little easier to follow. You only have to take a train to Kew Gardens in west London, home of The National Archives, to which MI5 periodic-ally releases documents (redacted where necessary) that are at least 50 years old and considered not to be inherently sensitive. There, you put in a formal request to view records and before long you are carefully opening a time-capsule box full of memos, minutes and correspondence. In January 2025, The National Archives received more than 100 such documents, including paperwork shedding new light on the discovery of the ‘Cambridge Five’ spy ring. Not long after, work began on The MI5 Archive: Official Secrets.
The book is the first visual record of the organisation’s activities and a remarkably evocative glimpse of a clandestine world. ‘It smells of card indexes and brush passes and last-minute saves,’ writes the novelist Nick Harkaway in his introduction. His father, John le Carré, began his intelligence career at MI5 and one of his untold stories is among those revealed here: during a training exercise, as part of which he had been instructed to ‘kidnap’ and interrogate a fellow officer, he accidentally snatched a local headmaster who happened to be cycling past.
The story of the Secret Service Bureau, as it was initially known, begins in August 1909, on Victoria Street in Westminster. Its remit, defined by the War Office and the Metropolitan Police, was to defend the country by both monitoring foreign agents in Britain and ‘running’ agents abroad. Domestic duties were given to what would later become MI5 and the ‘Foreign Section’, later MI6, looked after operations abroad.
Almost from the outset, the two branches of British intelligence were perceived as having distinct characters: one scholarly, one swashbuckling (to this day, MI5 staff are rumoured to refer to their opposite numbers as TSARs: ‘Those S***s Across the River’).
The First World War made for brisk business and, by 1918, MI5 was employing 850 staff, but with the disappearance of the German threat came swingeing budget cuts and soon there were only 30 people effectively camping out in a South Kensington townhouse. ‘To remain relevant to its Whitehall paymasters, a new enemy was required,’ notes The National Archives.
In 1920, there came the formation of the secretly Moscow-backed Communist Party of Great Britain. The ‘Defence Black List’ of left-wing individuals considered potentially injurious to the security of the state had 25,250 names on it by 1925, many of whom were having their telephones tapped and their post intercepted by an MI5 with a headcount of still only 35 people. The rise of Hitler meant all of this had to be put to one side. MI5 relocated to Wormwood Scrubs, with the cells converted into offices, and then, when the prison was bombed in the Blitz, Blenheim Palace in Oxfordshire.
Among the papers in The National Archives is a hair-raising September 1940 map showing the projected landings of Germany’s first-wave troop divisions in Operation Sea Lion. MI5, however, was remarkably successful in capturing the German agents sent to Britain and ‘turning’ them via the Double-Cross System, facilitating victory on D-Day. Wartime MI5 was full of eccentric characters such as Thomas Argyll ‘Tar’ Robertson, the remarkable Scottish intelligence officer and former Seaforth Highlander who sported tartan trews and went grouse shooting with the aid of a decoy bird of prey he had fashioned from umbrella ribs.
In 1945, MI5’s agent-runner Maxwell Knight — the complicated former jazz-band leader, Exmoor publican and early British Fascist said to have inspired Ian Fleming’s M — wrote that ‘in the history of espionage and counter-espionage a very high percentage of the greatest coups have been brought off by women’. MI5 gained its first female director general, Stella Rimington, in 1992 and female spies have been rightly celebrated in recent years. Yet there has always been an invisible army of women without whom this archive would not exist: the secretaries, shorthand typists and filing clerks who the then director general Vernon Kell addressed as ‘the ladies of the Office’ in a 1926 memo. As signatories of the Official Secrets Act, most never went on record to tell their stories—with a few exceptions.
The first mention of the Cold War reached the public in 1947 and fears around Stalin’s atomic ambitions led MI5 in some surprising directions. Records from 1948 held in The National Archives detail the activities of the Pigeon Sub-Committee, such as the minutes of a November 1948 meeting in which it was proposed ‘to see what happens to a homing pigeon when it flies over an atomic plant in full blast’.
Over several months, the scope of this inquiry expanded in increasingly Pythonesque dimensions, from the ‘feasibility [of] tying gadget to pigeons leg which would indicate whether bird passed near atomic-plant’ to the development of a container ‘for dropping pigeons from high speed aircraft’. Mention was also made of a pigeon-fancying member of MI5 staff who, if ‘assisted to maintain bigger loft than he does normally’, might ‘be able to meet modest calls for birds’ should a crack winged squad be required to see off the Soviet threat. A proposal for further research was made in May 1949 to the Atomic Energy Research Establishment at Didcot, Oxfordshire, but there seems to be no record of the pigeon test-flight ever taking place.
Within two years, things had taken a decidedly darker turn and then unknown, but now instantly familiar names were starting to appear in MI5 records. After Donald Maclean and Guy Burgess of the Foreign Office defected to Moscow in 1951, the organisation became suspicious about the activities of Harold ‘Kim’ Philby, an MI6 liaison officer at the British Embassy in Washington, D.C. He was recalled to London and interrogated by MI5 on suspicion of being a double agent, but always denied it, even calling a press conference in 1955 to protest his innocence.
On January 9, 1963, however, two days before his official confession statement, an internal MI5 memo records ‘that H. A. R. Philby has confessed’. Philby’s subsequent defection and the unmasking of fellow ‘Cambridge Five’ Soviet spy Anthony Blunt, then Surveyor of The Queen’s Pictures, were seismic events for the British intelligence community.
Yet although the MI5 archive contains plenty of transcriptions of phone-taps and surveillance logs, these years were far from a free-for-all. A November 1970 ‘Note For Record’ headed ‘Secret and Personal’ details the surveillance of the trade unionist Jack Jones, who was suspected of being an undercover Communist. ‘The Prime Minister agreed with the Home Secretary and the Foreign and Commonwealth Secretary… that the Security Service should be authorised to tap Mr Jones’s telephone. They should not instal a listening device in his flat without further Ministerial authority,’ it states.
Spying has always been an intensely bureaucratic business. ‘Why are we here?’ Harkaway asks in his introduction to the book — is it because we can’t get enough of ‘derring do’? ‘Do we want to know how spying is done to secure ourselves from it?’ he ponders. ‘Or to reveal the past?’ The whys and hows of spycraft certainly hold an enduring fascination, but it’s the quotidian details that really stick in your mind: tea-rings left on documents; terse marginalia; the sorts of small, but fiercely fought internal power struggles that will be familiar to anyone who has worked in an office.
The truth is that spies are rarely glamorous or particularly unusual — but it is this proximity of ordinary human foibles and failings to some of history’s most extraordinary moments that makes immersing yourself in the depths of the MI5’s evocative archive so compelling.