The news comes after a difficult 12 months for JWT, in which it lost its important Vodafone and Reckitt Benckiser accounts.
He joined the agency almost four years ago, hired by Simon Bolton, the London office's former chief executive.
Details have yet to emerge, but Craig Davis, global creative director of JWT, said: "We’re going to restructure the agency around groups, based on pools of our clients. The needs of our clients have changed over the past two years."
Bell, who was unavailable to comment, joined JWT from Leo Burnett. There he had earned a reputation for producing award-winning work on large, demanding accounts including McDonald's and Heinz. In 2001, his department won three Gold Lions in Cannes.
Mark Tutssel, his then partner and co-creative director, remained at Leo Burnett and is now the network's global creative chief.
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