The Post Office Tower, which is one of London's most recognised landmarks, has been given Grade II listed status.
The shape of the tower reduced wind resistance
The British Telecom Communication Tower was the first purpose-built tower to transmit high frequency radio waves.
It was designed to allow for the rapid expansion of telephone communications and to overcome the difficulty of laying cables in London.
Its construction began in 1961 but there were fears at the time that its height and modern style would be out of keeping with the tone of the area.
The cylindrical shape of the tower reduced wind resistance and gave it stability and style.
The tower, incorporating observation galleries and a restaurant, stands at 620 feet above street level, including a 40-feet high London Weather Centre radar mast.
But a terrorist bomb led to the closure of the observation galleries in 1971.
It was built by the Ministry of Public Building and Works between 1961-65
Plans for its creation date back to the 1936 Post Office Sites Act
It became operational on October 8, 1965 and was opened to the public in May 1966
It attracted 1.5 million visitors in its first year
The revolving segment spans 3 metres and completes a full circuit every 22 minutes
It cost £2.5m to build
The tower's two lifts travel at six metres a second and takes 30 seconds to reach the top
The restaurant closed in 1980 when the owner's lease expired.
It has subsequently been refurbished and is now used for corporate entertainment.
Its new status was granted by Sir Neil Cossons, the chairman of English Heritage, on Wednesday morning at the tower.
Arts minister Baroness Blackstone was also there.
She said: "It is an icon for sixties design and science, and we should be proud of it."
Listed buildings are graded according to their architectural or historical importance, with grade I denoting those of exceptional interest.
Grade II buildings are deemed particularly important, and like grade I buildings may be eligible for English Heritage grants for urgent major repairs.