NEW YORK (AP) _ The chief executive of J. Walter Thompson, the world's fourth-largest advertising agency, reportedly resigned on Thursday.

Chris Jones, 45, the youngest person to lead the New York-based ad agency, left unexpectedly for health reasons, The New York Times reported on its Web site on Friday. His resignation is effective immediately.

Jones, who has been on sick leave since the beginning of the year, will be succeeded by Peter A. Schweitzer, the Times reported. Schweitzer, long the agency's senior executive on its largest account, Ford Motor Co., will become J. Walter Thompson's eighth top executive since its founding in 1864.

J. Walter Thompson is owned by London's WPP Group.