The unremarkable, red brick building barely raised a glance from the hundreds strolling past its anonymous wooden doors every day.
But GCHQ has finally revealed that the nondescript office block on Palmer Street opposite St James’s Park Tube station in Westminster has been its London base for more than 65 years.
Hidden away behind its bland facade, some of the UK’s finest minds have been working to protect national security.
The organisation, long known as Britain’s listening post, has only now been able to confirm the location, codenamed station UKC1000, after it moving out of the base, which has been sold.
Jeremy Fleming, its director, 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.”