He will become non-executive chairman of the agency and will retain his duties until a replacement is found.

Peter Chadlington, CEO of Grayling owner Huntsworth, said: "Michael was a central architect of the successful strategy to merge many of our businesses together to form a single brand and attract larger, multi-office clients. And in doing so he has established a distinct culture and work style. Grayling is now established as one of the leading international public relations and government relations firms in the world.

"Michael has decided that it's the right time for him to develop a portfolio of interests. I am delighted that he will remain with Grayling as non-executive Chairman once a successor has been found, ensuring that the heritage and values which he has created will be carried forward and built upon by his successor. We have recently appointed head hunters but it is unlikely that the new CEO will be in post until sometime in 2013."

Michael said: "I am extremely proud of what we have achieved in Grayling over the past two and a half years since it became a global company. I want to continue to support the business, our clients and my colleagues but I also feel that the time is right to develop other interests which play to my many decades of international experience. I look forward to handing the baton over to my successor and to supporting him or her in the future."