Plessey

The Plessey Company plc was a British electronics, defence and telecommunications company. It was listed on the London Stock Exchange and was a constituent of the FTSE 100 Index until it was acquired by GEC and Siemens in 1989.

Plessey was established in 1917 with an initial focus on mechanical engineering; it diversified into electronics manufacturing in the 1920s, producing radios for Marconi and telephones for the General Post Office, along with various aircraft components during the 1930s. During the Second World War, Plessey's production capacity was used for the British war effort; it was during this time that it operated an underground factory in five miles of tunnels originally built for the London Underground. During the Cold War, the company expanded primarily via the acquisition of other companies as well as the formation of overseas subsidiaries. In 1961, Plessey merged with Ericsson Telephones and the Automatic Telephone Manufacturing Company to become the largest manufacturer of telecommunications equipment in Britain. Plessey played a role in the development of innovations such as the Atlas computer and System X digital telephone exchange. In 1988, Plessey and the General Electric Company (GEC) merged their telecom units to form GEC Plessey Telecommunications (GPT).

In 1989, the company was taken over by a consortium formed by GEC and Siemens, which split the assets of the Plessey group. The majority of Plessey's defence assets were amalgamated into BAE Systems in 1999 when British Aerospace merged with the defence arm of GEC, Marconi Electronic Systems. The Plessey Microsystems division was the subject of a management buyout in 1988 becoming , which survives today as part of based in Towcester, Northamptonshire. The bulk of Plessey's telecommunications assets were acquired by Ericsson through its 2005 acquisition of Marconi Communications, a successor company of GEC.

The Plessey brand continues to be used by a subsidiary of Haylo Labs Limited, which originated as Plessey Semiconductors and makes optical semiconductors.