The one-off documentary will air on ITV later this year.
Speaking to Dermot O'Leary whilst promoting their book Once Upon A Tyne, Dec revealed: "We've done a documentary - The Story Of SM:TV - it was lovely. We watched the first cut of it the other day, it's coming to ITV later in the year. It's the story of how it came about and we go into more depth about how we had the idea, we hear from David [Former controller of ITV], and then about how we met Cat and how she turned the tables on us and set us up in the first interview and absolutely roasted us. Then about how we got on air, and it was awful."
Last week, Cat Deeley revealed that her and the boys had filmed the documentary, which involved them returning to the studio, where the production team had recreated the original set.
The Story Of SM:TV will air later this year on ITV.