As reported on prweek.com (15 November), Martinson insisted that the move was a resignation and not part of the COI's widescale redundancy programme.
He said he was heading to 'new ventures', which would be revealed shortly.
Martinson said: 'I'm very proud of what our teams have achieved and the capability the COI now has in news and PR services across the country. COI is at another turning point and for me this is a good time to move on to new ventures. So I have decided to leave at the end of January.'
Martinson was previously director of comms at the Food Standards Agency and has worked for the BBC, Millennium Dome, Weber Shandwick, the Red Consultancy and Coca-Cola UK.