It is understood that Nick Theakstone, Group M UK's chief operating officer, will replace Allan as head of Group M. Schmidt-Vogel is believed to be moving to a trading position within the unit.
Before Group M, Allan was the UK chief executive of MediaCom for five years. When WPP moved MediaCom into its Group M buying unit in 2005, Allan was appointed chief executive of the group.
Shortly after his appointment, Allan created a new management structure at Group M, appointing Theakstone to the position of chief operating officer responsible for non-trading areas. Theakstone was formerly an investment director at MindShare.
Schmidt-Vogel has been MediaCom Worldwide's chief executive since 2002. Before that, he was the chairman and chief executive of MediaCom Europe.