Gayle King shared alarming details of an illness that left Oprah Winfrey in hospital.

The 69-year-old television host had been due to welcome Oprah, 70, into the studio on Tuesday's episode of CBS Mornings to celebrate the release of a new novel by David Wroblewski.

With Oprah absent to discuss the acclaimed author's new book, however, Gayle revealed her good friend had been rushed to hospital thanks to a vicious stomach illness.

Going into detail, Gayle told her alarmed guest, "I'm so sorry that Oprah can't (be here). Yesterday she said, 'I'm going to rally, I'm going to rally.' She had some kind of stomach flu where stuff was coming out of both ends.

"Needless to say, she ended up in the hospital, dehydration, and got an IV. So it was a very serious thing."

Giving more details on Oprah's condition, and realising she had perhaps said too much, Gayle added, "She's Ok. She will be okay. I hope she's not mad at me for sharing that detail, but I wanted to make it clear thought that it mattered to her and it really bothered her that she couldn't be here for you today... she so, so loved this book."

Oprah later took to social media to promote David's new book.

Posing with a copy, the talk show icon wrote, "Summer is here and I love to settle in with a big book that takes me through the season. So today I'm thrilled to announce that my next @oprahsbookclub selection is Familiaris by @davidw4words!"