The company said it had always been his intention to leave after the completion of the troubled company’s financial restructuring.
It says this was why he had taken the title of interim chairman when he was appointed in September last year.
A company spokesperson confirmed that Steve Hare remained as finance director and said there were no plans for him to leave.
The company has debts of around £4bn is negotiating with banks and bondholders over a debt-for-equity swap.
It is believed to owe about £2.2bn to and a further £2bn to bondholders.
A spokesman said: ‘When the financial restructuring is completed Derek Bonham will have completed his work for Marconi. Derek Bonham and Alan Thomas, a new non-executive director, have already started the search for a new chairman.’
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