New York ? Ted Koppel, who has provided a late-night alternative to laughs as anchor of ABC News’ “Nightline” since it began 25 years ago, said Thursday he would leave the network when his contract expired in December.

Koppel, 65, said he wasn’t retiring. His departure casts doubt on the future of “Nightline,” although Koppel and ABC News President David Westin expressed confidence that it will continue.

The broadcast’s longtime executive producer, Tom Bettag, will leave ABC News with Koppel.

Westin had made it clear that he wanted to expand “Nightline” to an hour and air live each weeknight (sometimes it’s taped). Koppel was offered the chance to continue, or switch jobs with Sunday morning’s “This Week” host George Stephanopoulos, but told Westin upon returning from a vacation this week that he wanted to leave.

“I would have preferred Ted to have stayed a few more years, but I respect his decision and I admire his courage to walk away,” Westin said.

Koppel and ABC News executives had worked out a transition plan when he signed his last contract five years ago, but it blew up in 2002 when ABC’s entertainment division made a secret bid to lure David Letterman from CBS. Letterman chose to stay, and the incident made “Nightline” employees question ABC’s commitment to their show.

Koppel said Westin had assured him he was not being pushed out the door.

“But who knows?” Koppel said. “Maybe it was. I’m too much a reporter and a realist, and have been in this business too long, not to recognize that my salary is very high, particularly for someone who only does three days a week now.”

He said he understood it was harder to keep an audience and make money in a fragmented television market where there are many more options than when he started “Nightline.” As a direct competitor to Letterman and NBC’s Jay Leno, the show’s viewership has dropped from a nightly average of 6.3 million a decade ago to 3.8 million this season, according to Nielsen Media Research.

“Nightline” began as a series of special reports during the Iranian hostage crisis in November 1979, and originally was anchored by Frank Reynolds. Then ABC News President Roone Arledge seized on the opportunity to wrest the time from affiliates, and it became a regular newscast the following March.