Boston Celtics center Al Horford will decline to exercise his $30.1 million option for the 2019-20 season and seek to leave the franchise as an unrestricted free agent this summer, league sources told ESPN.

Talks on a new deal with the Celtics have perished, with too great of a gulf between what the team is willing to offer and what Horford wants on a long-term deal, league sources said.

Celtics president of basketball operations Danny Ainge and Horford's agent, Jason Glushon, aren't expected to restart talks again, league sources said. Horford is entering free agency with the full expectation that his three-year Celtics career has ended, sources said.

Horford, a five-time All-Star, was a key member of the Celtics' core and a favorite of management, coaches and teammates.

Horford averaged 13.6 and 6.7 rebounds for the Celtics this past season, his third with Boston since leaving Atlanta for a four-year, $113 million free-agent deal with Boston. He has been to the Eastern Conference playoffs 11 times in his 12-year pro career. He won two national championships at the University of Florida.