Upon completion in 1964, it overtook the Millbank Tower to become the tallest building in both London and the United Kingdom, titles that it held until 1980, when it in turn was overtaken by the NatWest Tower.
Due to its importance to the national communications network, information about the tower was designated an official secret. In 1978, the journalist Duncan Campbell was tried for collecting information about secret locations, and during the trial the judge ordered that the sites could not be identified by name; the Post Office Tower could only be referred to as 'Location 23'.
The British Telecom microwave network (Backbone) was a network of point-to-point microwave radio links in the United Kingdom, operated at first by the General Post Office, and subsequently by its successor BT plc. From the late 1950s to the 1980s it provided a large part of BT's trunk communications capacity, and carried telephone, television and radar signals and digital data, both civil and military. Its use of line-of-sight microwave transmission was particularly important during the Cold War for its resilience against nuclear attack. It was rendered obsolete, at least for normal civilian purposes, by the installation of a national optical fibre communication network with considerably higher reliability and vastly greater capacity.
The term 'backbone' is often applied to the core of a communications network, i.e. the part that provides high-capacity links over long distances between major nodes. In the early 1950s, the term was used by the General Post Office (BT's predecessor) to describe a chain of microwave links designed to provide resilient communications in the event of nuclear war. It was originally designed as a chain of stations between south-east England and Scotland. The exact location of the Backbone sites changed as the project developed, but in July 1956 there were 14 planned sites at (from south to north).
BT remains one of the largest owners of transmission and microwave towers in the UK. The most famous of these is the BT Tower in London, which was the tallest building in the UK from its construction in the 1960s until the early 1980s, and a major node in the BT microwave network.
It is often said that the tower did not appear on Ordnance Survey maps, despite being a 177-metre (581 ft) tall structure in the middle of central London that was open to the public for about 15 years. However, this is incorrect; the 1:25,000 (published 1971) and 1:10,000 (published 1981) Ordnance Survey maps show the tower. It is also shown in the London A–Z street atlas from 1984.
In February 1993, the MP Kate Hoey used the tower as an example of trivial information being kept officially secret, and joked that she hoped parliamentary privilege allowed her to confirm that the tower existed and to state its street address.
It replaced a much shorter steel lattice tower which had been built on the roof of the neighbouring Museum telephone exchange in the late 1940s to provide a television link between London and Birmingham. The taller structure was required to protect the radio links' "line of sight" against some of the tall buildings in London then in the planning stage. These links were routed via other GPO microwave stations at Harrow Weald, Bagshot, Kelvedon Hatch and Fairseat, and to places like the London Air Traffic Control Centre at West Drayton.
The tower was designed by the architects of the Ministry of Public Building and Works: the chief architects were Eric Bedford and G. R. Yeats. Typical for its time, the building is concrete clad in glass. The narrow cylindrical shape was chosen because of the requirements of the communications aerials: the building will shift no more than 25 centimetres (10 in) in wind speeds of up to 150 km/h (95 mph). Initially, the first 16 floors were for technical equipment and power. Above that was a 35-metre section for the microwave aerials, and above that were six floors of suites, kitchens, technical equipment, a revolving restaurant, and finally a cantilevered steel lattice tower. To prevent heat build-up, the glass cladding was of a special tint. The construction cost was £2.5 million.
Construction began in June 1961; owing to the building's height and its having a tower crane jib across the top virtually throughout the whole construction period, it gradually became a very prominent landmark that could be seen from almost anywhere in London. A question was raised in Parliament about the crane, in August 1963. Reginald Bennett MP asked the Minister of Public Buildings and Works, Geoffrey Rippon, how, when the crane on the top of the new Tower had fulfilled its purpose, he proposed to remove it. Rippon replied: "This is a matter for the contractors. The problem does not have to be solved for about a year but there appears to be no danger of the crane having to be left in situ."
The tower was topped out on 15 July 1964, and officially opened by the then Prime Minister Harold Wilson on 8 October 1965. The main contractor was Peter Lind & Co Ltd.
The tower was originally designed to be just 111 metres (364 ft) high; its foundations are sunk down through 53 metres (174 ft) of London clay, and are formed of a concrete raft 27 metres (89 ft) square, 1 metre (3 ft) thick, reinforced with six layers of cables, on top of which sits a reinforced concrete pyramid.
