Cellnet

Telefónica UK Limited

Type	Subsidiary
Industry	Telecommunications
Predecessor	BT Cellnet
Founded	1985 (as Cellnet)
Founder	John Carrington
Headquarters	Bath Road, Slough, Berkshire, United Kingdom
Number of locations	484 stores (as of 9 January 2017)
Area served	United Kingdom
Key people	Mark Evans (CEO)
Revenue	£2.973 billion (2010)
Number of employees	21,580 (2013)
Parent	Telefónica
Subsidiaries	giffgaff
Website	www.o2.co.uk

Telefónica UK Limited (trading as O2 – stylised as O₂) is a telecommunications services provider in the United Kingdom, owned by the Spanish multinational Telefónica. It is the second-largest mobile telecommunications provider in the United Kingdom after EE Limited, and is headquartered in Slough.

O2 was formed in 1985 as Cellnet, a 60:40 joint venture between BT Group and Securicor. In 1999, BT Group acquired Securicor's 40 percent share of Cellnet and the company was later rebranded as BT Cellnet. In June 2000 BT Cellnet launched the world's first commercial General Packet Radio Service. BT Cellnet, together with BT Group's mobile telecommunications businesses in Germany, Ireland and the Netherlands, was part of the BT Wireless division. This was spun off from the BT Group in 2002 to form a new holding company, mmO2 plc, which introduced the new "O2" brand for the businesses. mmO2 plc was subsequently renamed O2 plc.

O2 plc was purchased by the Spanish telecommunications company Telefónica in 2006 for £18 billion. Under the terms of the acquisition, Telefónica agreed to retain the "O2" brand and the company's UK headquarters. O2 plc became Telefónica Europe.

Between 1985 and 1989, John Carrington was the CEO of British Telecom's Mobile Division and the Chairman of Cellnet. It was during this period that Carrington launched Cellnet's first cellular service, following the innovative development work by BT Spectrum, who initiated the chain of cells between London Heathrow and BT Tower in January 1985.

Cellnet was established in 1985 as a joint subsidiary of BTCR, British Telecom Cellular Radio, providing the engineering knowledge, and TSCR, Telecom Securicor Cellular Radio Limited, providing the financial investment, resulting in a 60:40 joint venture between British Telecommunications and Securicor.