BT Group plc (formerly British Telecom) is a British multinational telecommunications holding company headquartered in London, England. It has operations in around 180 countries and is the largest provider of fixed-line, broadband and mobile services in the UK, and also provides subscription television and IT services.

BT's origins date back to the founding in 1846 of the Electric Telegraph Company, the world's first public telegraph company, which developed a nationwide communications network. BT Group as it came to be started in 1912, when the General Post Office, a government department, took over the system of the National Telephone Company becoming the monopoly telecoms supplier in the United Kingdom. The Post Office Act of 1969 led to the GPO becoming a public corporation, Post Office Telecommunications. The British Telecom brand was introduced in 1980, and became independent of the Post Office in 1981, officially trading under the name. British Telecom was privatised in 1984, becoming British Telecommunications plc, with some 50 percent of its shares sold to investors. The Government sold its remaining stake in further share sales in 1991 and 1993. BT holds a royal warrant and has a primary listing on the London Stock Exchange, and is a constituent of the FTSE 100 Index.

BT controls a number of large subsidiaries. Its BT Enterprise division supplies telecoms services to corporate and government customers worldwide, and its BT Consumer division supplies telephony, broadband, and subscription television services in the United Kingdom to around 18 million customers.

A number of privately owned electrical telegraph companies operated in the United Kingdom of Great Britain and Ireland from 1846 onwards. Among them were:

The Telegraph Act 1868 passed the control of all these to the Postal Telegraphs Department of the newly formed General Post Office (GPO). The Telegraph Act 1869 granted the GPO a monopoly over communications.

With the invention of the telephone by Alexander Graham Bell in 1876 the GPO began to provide telephone services from some of its telegraph exchanges. It was confirmed in 1880 that the 1869 Act included telephony even though the telephone had not been invented when the Act was first conceived. In 1882, the Postmaster-General, Henry Fawcett started to issue licences to operate a telephone service to private businesses and the telephone system grew under the GPO in some areas and private ownership in others. The GPO's main competitor, the National Telephone Company (NTC), emerged in this market by absorbing other private telephone companies. It controlled most of telephony in Britain before the 1880 ruling on the Telegraph Act 1869 mandated a nationalised service – which was instated in 1911 prior to the absorption of the NTC into the GPO in 1912.

The trunk network was unified under GPO control in 1896 and the local distribution network in 1912. A few municipally owned services remained outside of GPO control. These were Kingston upon Hull, Portsmouth and Guernsey. Hull still retains an independent operator, Kingston Communications, though it is no longer municipally controlled.

The assets of the National Telephone Company were acquired by the UK Government to form Post Office Telephones in late 1911. Post Office engineers in the inter-war period had considerable expertise in both telecommunications and hearing assistive devices. Transistors were invented by Bell Telephone Laboratories in the US in 1948, however it was not until the mid-1960s that a transistorised oscillator was introduced to make the calling sound on a telephone in the UK.