Details of how a secret underground complex was created in case Westminster came under attack from the Soviet Union in the 1960s have come to light.
The bunker, code named Turnstile, included a telecoms unit, storage depots and a communications centre, with access to a range of military personnel at the height of the nuclear arms race.
The site - the location of which is now well known but is generally not available for day-to-day visits from the public - was declassified at the end of the Cold War, although it remains part of the Ministry of Defence estate.
Documents show the location of the Government war headquarters, in north-west Wiltshire, around 2km to the west of Corsham, was deemed far enough out of London to escape any attacks on the capital, but easy enough for Harold Wilson and his Cabinet to access.
Such was the secrecy surrounding Turnstile that only a handful of top-ranking officials knew of its existence, while senior Government press officers were issued with strict "cover stories" on how to deceive Fleet Street reporters.
Files from 1967 released by the National Archives show queries about the existence of the building were to be met with the flat response: "The installation has been developed to provide regional organisations in wartime with standby facilities for the maintenance of emergency services and public utilities.
"It also houses one of a number of Post Office centres for internal overseas communications in war.
"The remaining space has been made available to Government Departments for storage."
The briefing document also provided a modified cover story, outlining the site was a "defence installation for use in peace and war".