Duckworth, who has been with the agency since its formation 14 years ago, will remain at the agency for two days a week but is relinquishing his title, and the agency is buying back his shares.
He said: "I will now be known as plain Mr Duckworth and I am selling my shares as I think it is inappropriate to be representing others while still having shares in DFGW."
The agency has been preparing for the move for 12 months. While Duckworth was involved with planning for senior clients, the day-to-day running of the planning department has increasingly fallen to the agency's joint managing director, Hugh Cameron.
Duckworth, who makes the move from 1 July, has also stepped down from the IPA Council and other industry projects he has worked on.