[I]f I had written this post 18 years ago it is quite possible that MI5 would want to talk to me. It’s hardly the stuff of John le Carré but from the day it was built until Kate Howie MP, speaking in Parliament on 19th February 1993 spilt the beans by announcing to the public, and I quote:

Hon. Members have given examples of seemingly trivial information that remains officially secret. An example that has not been mentioned, but which is so trivial that it is worth mentioning, is the absence of the British Telecom tower from Ordnance Survey maps. I hope that I am covered by parliamentary privilege

when I reveal that the British Telecom tower does exist and that its address is

60 Cleveland Street, London (Hansard col.632).

The 621 foot high BT Tower was Britain’s most poorly kept secret. Londoners were expected to not notice its presence, in fact for many years it did not appear on any map as its location was protected by the Official Secrets Act, even the taking and storing photographs of the building was forbidden.

In a further secret twist Londoners seem to have been unaware of the changes that have recently been undertaken above their heads as engineers removed the 31 microwave dishes, once used to transmit top secret data across a nationwide network of similar towers. Right up until the 1980s, the microwave network was responsible for transmitting television signals and other data – some of it military. The arrangement comprised of a link of transmitters, stretched across the United Kingdom from north to south; with towers similar to the London GPO erected in Birmingham (at Snow Hill) and Manchester (in Heaton Park).

Being extremely secure, the system was also known by the codename, “Backbone” and, in the event of a nuclear attack, the resilient network would have provided vital communications for the government.

The tower is mostly circular because the designers noted that the only buildings that survived in Hiroshima and Nagasaki were round, with the shape allowing the enormous blast wave to surge round them. But considering the searing heat and 500mph blast wave unleashed by a nuclear weapon, it is doubtful that any buildings (or indeed people) would have been left standing.

The tower was conceived in the 1950s when broadband microwave technology seemed the best way serve the growing communication needs of the nation. It was designed to exchange microwave radio signals with other similar towers in locations such as Birmingham, Bristol and Portsmouth. Built in a yard off an existing telephone exchange, it was quite a neat engineering feat. A borehole survey revealed the hard chalk suitable for supporting foundations was 174 feet down, too far to be practically used. Instead an 88 foot square concrete “raft” was placed some 26 feet below street level, supporting a seven metre tall flat topped concrete pyramid, which in turn supported a hollow concrete shaft that forms the core of the tower. Even in 100mph winds it will not sway more than 7.4 inches. Swaying isn’t good for microwave transmission especially in a nuclear holocaust.

In 1962 the GPO Tower (as it was then known) overtook St. Pauls Cathedral as London’s tallest building, that title was briefly snatched away by the newly constructed Millbank Tower which took less time to build, but was regained when completed. It held that record until 1980 when the NatWest Tower (now renamed Tower 42) rose above the City skyline.

Known formerly as the General Post Office Tower its presence (or at least its purpose) might have remained a secret but for the fact of a restaurant which revolved every 22 minutes on the 34th floor which was operated by the holiday camp king Billy Butin. By 1971 the tower had been visited by over 5 million people, it only closed in 1980 amid security fears after a bomb had exploded in the gent’s toilet one night causing extensive damage which took two years to repair.

In defiance of the prohibitions placed upon acknowledging its presence it has appeared in BBC’s Doctor Who the War Machines which curiously does have a “D” Notice slapped on it as the YouTube clip has now been withdrawn by the BBC. The tower has been a popular backdrop to science fiction films among others V for Vendetta, The Fog, The New Avengers episode Sleeper, The Day of the Triffids and Harry Potter flies over it in a Ford Anglia. But the all time favourite the tower is featured in the most famous scene in The Goodies when it is toppled over by Twinkle the Giant Kitten in the episode Kitten Kong.

London’s secret tower is based upon an original post by Charlie on Cold War London.