The show will air live from its Chicago, Illinois, studio at 9 a.m. CT (10 a.m. ET), he said.
The show will end on September 9, 2011, as its 25th season draws to a close, according to a letter from Harpo, Inc., President Tim Bennett, addressed to partners and obtained by CNN affiliate WLS-TV.
"Tomorrow, Oprah will announce live on "The Oprah Winfrey Show" that she has decided to end what is arguably one of the most popular, influential and enduring programs in television history," Bennett wrote.
The show has been the highest-rated talk show for 23 consecutive seasons, according to the Oprah.com site. It is seen by an estimated 42 million viewers a week in the United States and is broadcast to 145 countries.
The show had its origins in 1984, when Winfrey moved to Chicago to host WLS-TV's morning talk show, "AM Chicago," which became the number-one local talk show, surpassing ratings for "Donahue," one month after she began, the site said. Within a year, it had expanded to an hour-long format and been renamed "The Oprah Winfrey Show." It entered national syndication in 1986.
In August 2004, Winfrey signed an extension for the show to keep it on the air through 2011.
"I got a call from Oprah and she told me that she is announcing that next year will be her last year," talk show host Ellen DeGeneres told her audience Thursday. "It will be her 25th year and she feels like it's time for her to stop.
"I don't think I could be here without her," DeGeneres said. "I think she has blazed a trial ... she is an amazing woman. She will always be the queen of daytime television ... she is fantastic and I love her and I wish her the best. She deserves to rest. She has worked really, really hard."
Over the years, "The Oprah Winfrey Show" has made news on several occasions. In September 2004, for instance, Winfrey began a new session by giving each member of the audience a brand-new car. In May 2005, Tom Cruise caused a sensation when he jumped up and down on Oprah's couch.
Other guests have shared their personal struggles, such as Elizabeth Edwards, wife of former vice presidential candidate John Edwards; performer Whitney Houston and Lisa Niemi, widow of actor Patrick Swayze.
The show has also served as a launching pad for several experts -- Dr. Mehmet Oz on nutrition and health; Suze Orman on financial matters; Dr. Phil McGraw on families and relationships; and Martha Beck, a life coach. McGraw has gone on to have a talk show of his own.
Along the way, Winfrey has revealed her struggles with weight loss and as a survivor of childhood sexual abuse. From the talk show, she has created the popular Oprah's Book Club; O Magazine; Oprah's Angel Network, a charitable foundation; and co-founded Oxygen Media, a 24-hour cable channel aimed at women.
In 2008, Winfrey announced that beginning in 2009, the Discovery Health Channel would be named OWN: The Oprah Winfrey Network. The network is "a multi-platform media company designed to entertain, inform and inspire people to live their best lives," according to Oprah.com. Harpo Productions has also grown to include Harpo Films.
OWN is scheduled to make its cable premier in January 2011, according to a senior executive with the cable channel.
Winfrey's wealth was estimated at around $1 billion by Forbes in 2004. Last year, she topped the Forbes Celebrity 100 list that measures both power and money.
-- CNN's J.D. Cargill contributed to this report
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