(CNN) -- Oprah Winfrey will announce on Friday's "The Oprah Winfrey Show" that she will end her talk show, said a spokesman for Winfrey's Harpo Productions.

"Oprah will be ending her talk show," Harpo spokesman Don Halcombe said Thursday. "She will be speaking about it on tomorrow's live show."

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. that 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.

Marquee Blog: Oprah to make it official

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 No. 1 local talk show. The show surpassed ratings for "Donahue" one month after it 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 trail. ... 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 show has made news on several occasions. In September 2004, for instance, Winfrey began a new season by giving each member of the audience a new car. In May 2005, Tom Cruise caused a sensation when he jumped up and down on Oprah's couch, declaring his love for actress Katie Holmes.

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.

Read Harpo's letter to partners obtained by CNN affiliate WLS

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 went 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.

Check out Oprah's Book Club web event on CNN

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.

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.