She said: "Being in the Spice Girls was a hugely important part of my life and I wish my girls so much love and fun as they go back on tour, I know they will put on an incredible show and the fantastic fans past and present are going to have a wonderful time!"
She added, to Theakston: "Please don't look at me any more, or I might just blurt it out".
Ahead of the announcement, a new image of the Spice Girls was released.
This will be the first time Emma, Geri, Mel B and Mel C perform without Victoria, who is thought to be concentrating on her fashion business.More news: Khachanov revels in 'biggest achievement' after Djokovic upset
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They performed as a four-piece for two years after Horner quit the band in 1998, split in 2000 and last reunited at the 2012 London Olympics.
Spice Girls Tour 2019 tickets go on sale Saturday 10th November at 10.30am.
"I don't think anyone knows that publicly but I don't mind taking the blame, it was my fault so I was determined to get us back together and I'm still determined it will be five at one point". At the time, sources said she'd given in, and the tour was a go.
Asked if they'll go outside of the United Kingdom, she said: "We are".
The group is set to reveal all the details of the tour around Monday morning, but Baby Spice. "We all agree that there are many exciting possibilities that will once again embrace the original essence of The Spice Girls, while reinforcing our message of female empowerment for future generations". It was previously known that she would not be joining her former bandmates for their comeback tour.