His departure is the first change instigated in the network by the new Euro RSCG Worldwide management team, which took over in July.
The decision follows the loss last week of the £35m Argos account to Clemmow Hornby Inge and Euro RSCG's removal from the COI roster last month.
However, despite these and other losses such as British Heart Foundation and Christian Aid, Langdon brought in new-business wins including the £19m News International and the £10m Superdrug businesses.
However, other factors, including a culture clash with the network over his management style, are also believed to have played a major role in his removal.
Speculation about Langdon's future has been mounting since Jim Heekin, the Euro RSCG Worldwide chief executive, left when he was overlooked in a management restructure in August. Heekin, who hired Langdon to the agency in 2004, was immediately replaced by David Jones, chief executive of Euro RSCG New York.
Langdon said: "I wish them all the best of luck. I enjoyed my time at the agency immensely. I came to work for Jim Heekin and the agency has won more business in the last year than it probably has in the previous four.
"Heekin's exit meant that my departure was probably inevitable given that I came in as part of his management team. I'm proud of the creative work we produced and the work we've won. It was a privilege to work with the likes of Richard Kelly, Naomi Troni and Gerry Moira, who will continue to run the agency."
No replacement for Langdon has yet been lined up. In the interim, Chris Pinnington, the UK chief executive of Euro RSCG Worldwide, will run the agency.
Jones confirmed Langdon's exit, but could not comment further for legal reasons.
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