NEW YORK—Willis Group Holdings Ltd. said Friday that it has begun a search for a new chief financial officer to succeed Patrick C. Regan early next year.

Mr. Regan, who also is Willis’ group chief operating officer, is leaving Willis to join London-based Aviva P.L.C. Mr. Regan will stay with Willis’ operations in New York through the end of this year and assist in the search for his successor, the brokerage said in a statement.

Chicago-based executive consulting firm Spencer Stuart is assisting in the search, Willis said.

Mr. Regan joined Willis as an executive officer in January 2006. He has been CFO of the brokerage since March 2006 and group COO since 2008.