James is leaving after failing to agree on the future direction of the business with Naked Communications' founders.
Geoff Gray, who joined the business in January from Branded Communications, will take over the operation.
James quit his job as the managing director of Mediaedge:cia to launch Naked Ambition at the end of last year. He had only joined MEC four months previously.
The new agency was set up to help Naked grow without losing its small-agency culture and also had the potential to operate as a conflict shop.
The model was championed by Garry Lace, then the Grey chief executive. However, the scope for the agency's growth was predicated on Grey's investment. When Lace parted company with Grey earlier this year, the prospect of the Grey investment dried up.
Naked denied the agency would close and said that Gray is working on a new strategy for the venture. James will remain at the agency in the short term to hand over clients such as Air Miles, Ben Sherman and Universal Pictures' DVD business.
Separately, Naked has held talks with Bruno Kemoun and Eryck Rebbouh about setting up a strategic communications arm for their KR Media start-up. The two parties have been in discussions about creating an offering, which would be based in France. The former Aegis Media Europe joint chief executives open for business next month.
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