The BT Tower is a communications tower located 60 Cleveland Street, Fitzrovia, London, United Kingdom owned by BT Group, constructed from 1961 to 1964. It has been previously known as the Post Office Tower, the London Telecom Tower and the British Telecom Tower. The main structure is 177 metres (581 ft) tall, with a further section of aerial rigging bringing the total height to 189 metres (620 ft). Instruments are mounted near the top of the tower to facilitate long-term observations of London’s atmosphere. OS Grid Reference TQ2910581982.

The BT Tower housed a suite of air quality monitoring instrument during the ClearFlo (Clear air For London) campaign from 2011 to 2013 located on the 35th floor - hence the "t35" designation used in the filenames referring to this location.

The tower continues to host a number of instruments for long-term air-quality monitoring over London.