A line of computers, and a company, of historic importance in the British computing industry. J. Lyons & Co (a large firm in the catering industry) initiated in 1947 a project to build a computer to mechanize clerical functions in their own offices. (This decision was almost simultaneous with a similar decision in the US by Eckert and Mauchly, which led to UNIVAC 1.) The project was led by T. R. Thompson, a mathematician, and J. Pinkerton, an electrical engineer. The machine they built, LEO (Lyons Electronic Office), was fully operational at the end of 1953.

In 1954, Leo Computers Limited was founded. The company traded until 1963, when it was merged with the computing division of English Electric. During that time it marketed the LEO III, an extremely advanced commercial machine for its time. See also ICL.