Oprah Winfrey began her show in 1984 as 'A.M. Chicago', before it was changed to 'The Oprah Winfrey Show' a year later. (AP)

Oprah Winfrey was set to announce Friday that her powerhouse daytime television show, the foundation of a multibillion-dollar media empire with legions of fans, will end its run in 2011 after 25 seasons on the air.

Winfrey planned to announce the final date for “The Oprah Winfrey Show” during a live broadcast, according to her production company, Harpo Productions Inc.

Once a local Chicago morning program, the production evolved into television’s top-rated talk show for more than two decades, airing in 145 countries worldwide and watched by an estimated 42 million viewers a week in the U.S. alone.

“Oprah Winfrey is in a category of her own,” said Robert Thompson, professor of television and popular culture at Syracuse University. “This is a great American story and like any great American story it’s supersized.”

A Harpo spokeswoman declined to comment Thursday on Winfrey’s future plans except to say that “The Oprah Winfrey Show,” which has seen ratings slip 7 percent from a year ago, will not move to cable television.

Winfrey, 55, is widely expected to start up a new talk show on OWN: The Oprah Winfrey Network, a much-delayed joint venture with Discovery Communications Inc. that is expected to debut in 2011. OWN is to replace the Discovery Health Channel and will debut in some 74 million homes. An OWN spokeswoman declined comment Thursday.

CBS Television Distribution, which distributes “The Oprah Winfrey Show” to more than 200 U.S. markets, held out hope it could continue doing business with Winfrey, perhaps producing a new show out of its studios in Los Angeles.

“We know that anything she turns her hand to will be a great success,” the unit of CBS Corp. said in a statement. “We look forward to working with her for the next several years, and hopefully afterwards as well.”

Winfrey’s 24th season opened this year with a bang, as she drew more than 20,000 fans to Chicago’s Magnificent Mile for a block party with the Black Eyed Peas. She followed with a series of blockbuster interviews — Mike Tyson and Evander Holyfield, exclusives with singer Whitney Houston and ESPN’s Erin Andrews, and just this week, former Alaska governor, GOP vice presidential candidate Sarah Palin.

As a newcomer, “The Oprah Winfrey Show” chipped away at talk king Phil Donahue’s dominance. Later, turned to inspiration. The show covered ranged from interviews with the world’s celebrities to an honest discussion about Winfrey’s weight struggles.

“As the show evolved, it really kind of dressed up the neighborhood of the daytime talk show,” Thompson said.

In 1986, pianist-showman Liberace gave his final TV interview to Winfrey, just six weeks before he died. In a 1993 prime-time special, Michael Jackson revealed he suffered from a skin condition that produces depigmentation. Tom Cruise enthusiastically declared his affection for the much-younger Katie Holmes on the program in 2005 — and jumped on the couch to prove it.

In 2004, Winfrey unveiled her most famous giveaway, when nearly 300 members of the studio audience opened a gift box to find the keys to a new car inside. The stunt became a classic show moment as much for Winfrey’s reaction — “You get a car! You get a car! You get a car! Everybody gets a car!” — as its $7 million price tag.

The show also became a launching pad for Oprah’s Book Club, which then launched best-sellers. The titles ranged from “Song of Solomon” and “Paradise” by Toni Morrison to Wally Lamb’s “She’s Come Undone” and Elie Wiesel’s “Night.”

For others, the selection backfired. “A Million Little Pieces” exploded in sales after Winfrey chose the James Frey memoir in fall 2005. Soon after, it was revealed as a fabricated tale of addiction and recovery, and Winfrey later chewed out Frey on her show.

“I call her ‘Queen of the New Consciousness’ because she did so many things to change lives, the books that she promoted,” said hip-hop mogul Russell Simmons.

The loss of “The Oprah Winfrey Show” would be a blow to CBS Corp., which earns a percentage of hefty licensing fees from TV stations that use it — largely ABC affiliates. CBS Chief Executive Leslie Moonves told analysts two weeks ago that the contract with the show runs through most of 2011 and “if there’s a negative impact, it wouldn’t hit us until ’12.”

“Oprah’s been a force of media and there’s really no person you can look to out there who you could say, ‘That’s the heir apparent,’” said Larry Gerbrandt, an analyst for Media Valuation Partners in Los Angeles. Gerbrandt noted many stations build their schedules around Winfrey’s show.

“It’s a big loss, but not as huge as it would have been 10 years ago,” he said. “However, it still commands the biggest audience and ABC station competitors are licking their chops.”

Talk of the show’s end often has accompanied Winfrey’s contract negotiations. Before signing her current contract in 2004, she talked about quitting after the 2005-2006 season. As far back as 1995, she called continuing “a difficult and important decision.”

Winfrey started her broadcasting career in Nashville, Tenn., and Baltimore, Md., before relocating to Chicago in 1984 to host WLS-TV’s morning talk show “A.M. Chicago” — which became “The Oprah Winfrey Show” one year later. She set up Harpo the following year and her talk show went into syndication.

Powered by the show’s staggering success, Winfrey built a media empire. Harpo Studios produces shows hosted by Dr. Phil McGraw and celebrity chef Rachael Ray. O, The Oprah Magazine was the nation’s 7th most popular magazine in the first half of 2009.

“I came from nothing,” Winfrey wrote in the 1998 book “Journey to Beloved.” ”No power. No money. Not even my thoughts were my own. I had no free will. No voice. Now, I have the freedom, power, and will to speak to millions every day — having come from nowhere.”

Earlier this year, Forbes scored Winfrey’s net worth at $2.7 billion.