Neither would comment on the plans, but sources suggest the start-up will not be a digital specialist or a direct rival to Tribal DDB, but will work across a wide range of media.
Mike Parsons, the head of Tribal DDB Amsterdam, will replace Brock as the managing director of Tribal DDB. He starts in November. Dan Ng, the head of integration at DDB London, has been made the agency's planning director with immediate effect.
Brock and Dyke have worked together since 2005, when Brock joined Tribal from Agency.com. Dyke, formerly the planning director of Tribal, was made the planning director of DDB London when planning was integrated across the group agencies.
Stephen Woodford, the chief executive and chairman of DDB, said: "We will be sad to see Bill and Matt leave."