Davis commented: "After Simon Bolton arrived it was always my intention to get back to what I was doing before Stephen Carter left."
Bolton, the chief executive, joined JWT last year from FCB San Francisco. The post had been vacant since the departure in 2000 of Carter, the previous chief executive.
Michael Maedel, the head of JWT Europe, said that there was no immediate need to replace Davis and that the chairman's job would be kept open "for the time being".
Davis devoted one day a week to his chairman role, which he took up in 1998. He will continue to run the Shell Oil and Unilever accounts globally, but will drop his local responsibilities.
Davis confirmed: "The Unilever role is expanding to take into account the relationship with all the WPP agencies."
His new WPP role is part of a new initiative from the holding company which will see its networks working more closely together. Luis Bassat, the chairman of Bassat Ogilvy in Spain, takes on the chairman of WPP Iberia title, while William Lo becomes the chairman of WPP China.
Separately, Maedel denied reports that David Lamb, JWT's global planning director, is returning to London. Lamb moved to JWT New York last June.
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