Today Show airs without Lisa Wilkinson

Wilkinson will become part of The Project - among other roles - from January 2018 after spending 10 years as Today show host.

Speaking on Today early on Tuesday morning, co-host Karl Stefanovic briefly addressed his co-host's departure.

"A very big day for us on the show and a very big day for Lisa Wilkinson ... plenty more to say about that later in the program," Stefanovic said at the start of the show at 5.30am.

Deb Knight is filling in for Wilkinson.

Stefanovic and Wilkinson had formed a formidable ratings duo in recent years, with Today becoming the most watched breakfast program in major cities in 2016.

In a statement, sent at 8:43pm on Monday night, the Ten network said: "Lisa will join the team of Ten's award-winning news and current affairs program, The Project, in a hosting role.

"She will work alongside The Project's hosts Carrie Bickmore, Waleed Aly and Peter Helliar, as well as hosting The Sunday Project."

The decision by Wilkinson, 57, is understood to have shocked executives at Nine.

Ten is being bought by US corporation CBS and is understood to be keen to challenge Seven and Nine in the ratings.

The Project was recently expanded to six nights a week to include a Sunday edition.

Ten's chief content officer, Beverley McGarvey, said: "Lisa Wilkinson is one of the most respected journalists and television presenters in Australia and we are thrilled that she has agreed to join Ten's fantastic stable of on-air talent."

Minutes before the Ten announcement Wilkinson tweeted a statement from Nine: "[We] confirmed we have been unable to meet the expectations of Lisa Wilkinson and her manager on a contract renewal for a further period. We express our gratitude to Lisa for her 10 years with the Today show and are disappointed we find ourselves in this position.

Wilkinson said: "I want to thank Channel Nine for the privilege of co-hosting the Today show for the last 10 years.

"I would particularly like to acknowledge my colleagues Karl, Sylvia, Georgie, Tim, Richard, Stevie and Natalia, as well as all the hardworking producers and the entire team behind the cameras - every one of them has contributed to the incredible success of the show over the years."

Wilkinson was the youngest editor of Dolly magazine in the 1980s aged 21. She went on to edit Cleo and then had roles on television programs Beauty and the Beast, The Morning Shift and Weekend Sunrise.

Wilkinson was lauded by viewers and welcomed and congratulated by her peers following news of her departure.