In September 1968 the  merger occurred between the rapidly expanding GEC and English Electric. During December 1968 the GEC, AEI, English Electric and Elliott-Automation  electronics companies were forged into two major groups. The two groups were GEC-Elliott-Automation and GEC-Marconi Electronics which incorporated Elliott Flight Automation. Aerospace and defence were identified as key growth areas. Following GEC's acquisition of Marconi as part of English Electric in 1968 the Marconi brand was used for its defence businesses e.g. Marconi Space & Defence Systems (MSDS), Marconi Underwater Systems Ltd (MUSL).

In September 1969 ‘FLIGHT International ‘carried another full page family tree which commenced with ‘The General Electric and English Electric companies Ltd’. This  flowed down to GEC-Marconi Electronics Ltd of Marconi House, Chelmsford, Essex, which had a total group employment of 30,000. One branch then went to Marconi-Elliott Avionic Systems Ltd at Airport Works Rochester.

Marconi Electronic Systems (MES), or GEC-Marconi as it was until 1998, was the defence arm of The General Electric Company (GEC). It was demerged from GEC and acquired by British Aerospace (BAe) on 30 November 1999 to form BAE Systems.