It was founded 112 years ago amid escalating tensions with Imperial Germany.
Since then, MI5's spies have worked in the shadows to snoop on targets, foil bomb plots and keep leading politicians informed.
But now, the domestic intelligence agency, which is also known as the Security Service, has said goodbye to secrecy and revealed the inside of its former Mayfair base for the first time.
Posting the 'for your eyes only' pictures on it's Instagram account, the spy agency said: 'More than 25 years after MI5 left the building for the last time, we're now pleased to be able to share some rare images of our Curzon Street office, the home of MI5 between 1977-1994.'
The collection of five photos shows female intelligence officers at their desks at 1-4 Curzon Street, known as 'Curzon Street House', where the Security Service housed its registry, administration and technical services departments.
The female staff were known as 'Registry Queens' and it was here that phone-tapping and other electronic surveillance methods were monitored.
MI5 has taken to instagram to share images of its former headquarters in Curzon Street, Mayfair. Above: A female member of staff sits at a machine in the undated photo
The collection of five photos shows female intelligence officers at their desks at 1-4 Curzon Street, known as 'Curzon Street House', where the Security Service housed its registry, administration and technical services departments. MI5 vacated the building in 1994
In one photo, a dark-haired spy is seen listening back to a reel of tape, presumably of a covert surveillance.
Another photo shows an office with women apparently listening to recordings.
A third shows a female officer changing a spool of tape on a machine.
Built in the 1930s, the most noticeable feature of the building was the absence of windows on the street level.
Higher floors were hidden by weighted net curtains to prevent prying eyes.
In one photo, a dark-haired spy is seen listening back to a reel of tape, presumably of a covert surveillance
Another photo shows an office with women apparently listening to recordings
A third image shows a female officer changing a spool of tape on what appears to be a recording device
Writing on Instagram, MI5 added: 'Although there was never any official mention of MI5's offices at Curzon Street, bus conductors and taxi drivers were far less
