A drab office block sandwiched between a pub and a branch of Starbucks was a secret base of spy agency GCHQ, it has been confirmed.
The anonymous building opposite St James's Park Tube station in central London was used by British spooks for 66 years.
Despite the covert goings-on within, neighbours said the address's purpose was an open secret among locals.
GCHQ acknowledged the location after moving out of its home.
Director Jeremy Fleming said the site in Palmer Street, used by intelligence officers since 1953, had been part of "a history full of amazing intelligence".
GCHQ, known as Britain's listening post, was set up on 1 November 1919 as a peacetime "cryptanalytic" unit.
During World War Two, staff were moved to Bletchley Park to decrypt German messages including, most famously of all, the Enigma communications.
When the service moved its headquarters to Cheltenham from the London suburb of Eastcote in the 1950s, the Ministry of Works provided the Palmer Street building as a centre to handle secret paperwork and a base for its director.
'It was just common knowledge'
By Joseph Lee, BBC News
It turns out that it's hard to put a spy HQ with frosted glass windows in the middle of central London without someone asking questions.
And at the Adam and Eve pub at the end of Palmer Street, the answers were surprisingly accurate. Asked if she knew what the building next door was, a barmaid said: "It's M… MI6?"
The secretive neighbours were just a fact of life and she could not remember how she found out, she said. "We don't really talk about it."
She suggested that the landlord might know more, but the BBC's inquiries fell foul of the pub chain's operational security - press enquiries have to go through head office, the landlord said, before disappearing through a door marked "The Hideaway".
Staff at Starbucks on the other side of the office building either knew less or refused to crack under interrogation. Regina Toth, who had worked in the coffee shop for two years, said: "It was very mysterious."
At Pall Mall Barbers, Jack Holden knew it was "secret service or something". He'd worked opposite the GCHQ building for six years, but seeing police cars come with lights flashing to move on a loiterer in the street convinced him that "it's legit".
"It was just common knowledge," he said. "It looks suspect, doesn't it? With those blacked-out windows. It's a very secure building."
The spy agency said the unremarkable building, which is between a coffee shop and the Adam and Eve pub, had "played its part in significant events over the years, such as the 2012 London Olympics".
It has been sold privately but future plans for it are not currently known.
Despite the sale, GCHQ said it would maintain a presence in the capital, in addition to its Cheltenham HQ and other offices in Bude in Cornwall, Scarborough, Lincolnshire and Harrogate.
A new secure facility is also set to open in Manchester later in the year.