Sandwiched between a coffee shop and a pub, an unremarkable Westminster office block has had its secret unmasked.
For the last six decades the building, on Palmer Street and opposite St James's Park tube station, has been the London home of spy agency GCHQ.
Unknown to the public, intelligence officers have worked to protect national security from the drab-looking building since 1953.
It has been at the centre of planning for key events including the London 2012 Olympics.
The inconspicuous-looking red brick building has also been the control centre from which counter-terror operations have been mounted.
GCHQ director Jeremy Fleming said: “As we depart our Palmer Street site after 66 years, we look back on a history full of amazing intelligence, world-leading innovation, and the ingenious people who passed through those secret doors.
“Then, as now, it’s a history defined by the belief that with the right mix of minds, anything is possible.”
The true use of the spy facility has only been revealed after the agency moved out to a new base - the location of which is, of course, top secret.
What is GCHQ and what does it do?
GCHQ works alongside MI5, MI6 and law enforcement to protect the UK from threats including cyber attacks and terrorism.
The spy agency was established on November 1 1919 as a peacetime “cryptanalytic” unit made up from staff from the Admiralty’s Room 40 and the War Office’s MI1(b).
During World War Two, GCHQ personnel moved to Bletchley Park where they decrypted German messages, most famously by breaking Enigma communications.
In the early 1950s, the service moved its headquarters from the London suburbs of Eastcote to Cheltenham, meaning a suitable location was needed in the centre of the capital for the handling of secret paperwork and as a regular base for its director.
After a search, the Ministry of Works provided a newly built, Government-leased building on Palmer Street, where it stayed until it moved earlier this year.
The agency, which is celebrating its centenary, now has bases in Devon, Yorkshire and Gloucestershire.