LEO III was the solid-state successor to the earlier LEO I/II valve-based range. First built in 1961, some 61 machines were made some surviving into the 1980s.

The LEO company was sold to English Electric in 1963 and thus became part of ICL in 1968. Production ceased in 1966 but was restarted in 1969 at the request of the GPO. In 1981, the life of the LEO III system was further extended by the implemention of LEO III microcode on the ICL 2960 computer. The principal programming language was CLEO a propietary equivalent of COBOL.

No LEO computers have survived though small artefacts are held by museums and individuals.

	to David Holdsworth’s LEO III web pages to find a comprehensive description of the LEO III software restoration project. In addition to an emulator (Windows, Linux, OSX) there is a good selection of original source code and the source code of the emulator itself written in C.