Phil Kent, chairman and chief executive officer of Atlanta-based Turner Broadcasting System Inc., told colleagues Wednesday he will not renew his contract with the company when it ends next year.

In a letter obtained by The Atlanta Journal-Constitution, Kent said his decision followed many conversations with Time Warner CEO Jeff Bewkes over long-term leadership planning for Turner, a company started by cable TV pioneer Ted Turner but is now a division of Time Warner.

Chief Financial Officer John Martin will assume the title of CEO at Turner in January. Kent will remain chairman during a transition period next year

“Nothing changes for me or any of you until then,” Kent told colleagues. “ I’ll come in tomorrow and be on the job with the same enthusiasm as today, just as I know you will.“

Bewkes praised Kent’s leadership at Turner, which also operates CNN, TBS, TNT, Turner Classic Movies, Cartoon Network and truTV.

”At Phil’s initiation, he and I agreed that now is the right time to announce the next generation of leadership, and I am pleased that he will serve as chairman,” Bewkes said.

Bewkes credited Kent with guiding Turner’s networks “through a period of rapid change, all the while building on each network’s leading position with viewers and advertisers and forging stronger relationships with distributors.”

In an effort to reverse poor ratings at CNN last fall, Kent hired former NBCUniversal President Jeff Zucker in November to replace Jim Walton as head of the struggling Atlanta based cable news network. Bewkes reportedly had ordered the network’s top brass to find a solution to its programming issues. While at NBC, Zucker was credited with building up NBC’s “The Today Show” as executive producer in the 1990s.

Kent has been chairman and CEO since February 2003. He joined Turner in 1993 from Creative Artists Agency. Over the years has served as president and chief operating officer of CNN News Group, president of Turner Broadcasting System International and president of Turner Home Entertainment.

In his letter, Kent said “a dozen years is a long run in a top leadership position, particularly in such a dynamic and fast-changing industry as ours. I’m very proud of Turner’s business results over the past decade. But I’m equally proud of the culture that we have built together to foster creativity, collaboration and risk-taking in the pursuit of excellence.”

Before being named CFO in 2008, Martin had been executive vice president and CFO of Time Warner Cable Inc. since August 2005. Before joining Time Warner Cable, he spent nearly 12 years at Time Warner, most recently serving as senior vice president for investor relations from 2002 to 2005.