Cost£2.5m (£44m today)
Build a tower to relay telecommunications from London to the rest of the country
The Post Office tower – now known as the BT Tower – opened in the Fitzrovia area of central London in 1965.
The tower's main structure was 581ft (177m) high. A further section of aerial rigging brought the total height to 627ft (191m). It was the tallest building in the UK until London's NatWest Tower opened in 1980.
The tower was designed to relay the microwave signals used to carry telecommunications from London to other parts of the country. Its height was needed to protect the radio links' 'lines of sight' against some of the tall buildings that were then planned for London.
As well as the microwave relays, technical equipment and offices, the tower was also famous for its revolving restaurant on the 34th floor.
The restaurant took 22 minutes to rotate fully and was run by Billy Butlin, best known for his chain of holiday camps across the UK. It closed in 1980 amid security fears.
The building quickly became a London landmark, although its existence was – bizarrely – an official secret. MP Kate Hoey drew attention to this odd contradiction when she mentioned the structure during a parliamentary debate in February 1993.
The tower was made a Grade 2 listed building in 2003.
Post Office Tower
Graduate engineer at Transport for London, Louis Watson, talks to us from the top of the Post Office Tower, more commonly known today as the BT Tower.
The tower was designed to relay the microwave signals used to carry telecommunications from London to other parts of the country. Its height was needed to protect the radio links’ ‘lines of sight’ against some of the tall buildings that were then planned for London.
Did you know …
The tower's lifts can go from ground level to the top in 20 seconds. Moving at 7m a second, they're among the fastest in Europe.
Most of the tower's original microwave aerials have been removed as digital transmissions replaced microwave.
The tower now houses a Mediahive digital content management system with a storage capacity of 3.6 petabytes (3,600 terabytes).
Difference the tower has made
The tower was an important part of the microwave relay that was the backbone of the UK's telecommunications network at the time. It could handle up to 150,000 telephone conversations and 40 television channels when it opened.
The structure quickly became a symbol of London, both in the UK and overseas – despite its 'officially secret' status.
It featured as a key location in BBC TV's 'Doctor Who' within a year of opening. It also appears in the 1967 Peter Cook and Dudley Moore film 'Bedazzled' and the 2005 film 'V for Vendetta', as well as many other film and TV appearances.
How the work was done
The 191m tower is circular and made of concrete covered in glass.
The narrow cylindrical shape was chosen because of the requirements of its communications aerials: it was designed to shift no more than 10 inches (25cm) in winds of up to 95mph (150km/h).
Surveyors working on the project found the hard chalk they needed to support the foundations was over 170ft (52m) down – too deep to be useful.
The project team sank a concrete raft about 8m into the ground to deal with this. The 27m² raft was 1m thick and reinforced with 6 layers of cables.
Engineers built a reinforced concrete pyramid on top of the raft. The pyramid supports a hollow concrete shaft that forms the core of the tower.
The tower used specially tinted glass cladding to prevent the building getting too hot.