The position of chief executive will not be replaced and the collection will be managed by the board of directors, with direction from the marketing committee. Church House Conference Centre general manager Robin Parker will also join the board as a new director.
Dee, who will retire on his 66th birthday, was an original founder of TWC, which counts The London Transport Museum and Central Hall among its 53 member venues and 36 associate and affiliate members.
The network reports that its member venues doubled from 25 to 50 last year. It aims to drive sales leads, share referrals amongst its members and promote Westminster as a business destination.
In a letter to TWC member venues, Dee wrote: "It has been a great privilege, and tremendously satisfying, to have been able to drive TWC from its small beginnings to its current position as a leading venue marketing collective in London. I am proud of the success achieved."
"Along the way I have been supported by many who shared my vision, and who helped to create a model that has been adopted by others in their search to represent their own niche destinations better, and to drive more business into their exceptional venues."
He said he is looking forward to spending more time with his family, walking and "learning to cook".
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