Simon Cowell says Adele agreeing to perform during the X Factor final is "the best news I've had all year".
Adele will sing on Sunday night's show, as will Coldplay, One Direction, Little Mix, Fleur East, Ben Haenow and Leona Lewis.
Cowell said: "It took me a while to make up my mind whether we really wanted her or not ... are you kidding?
"When we got the news and I had to keep quiet last weekend! This is the best news I've had all year.
"She is absolutely going to cap off what I hope is going to be an amazing weekend, and actually it's really the best of British."
Cowell also said he was not worried about The Voice transferring to ITV - the same channel as The X Factor.
Speaking at a red carpet event in Wembley ahead of the final, he said: "It doesn't matter to me whether it's BBC or ITV.
"I think in a weird way it will be a little bit more gentlemanly when it's on ITV, because I've been on lots of networks where we have two shows at the same time - and it (The X Factor) is going to stay in the same place, they've already told us that."
But he confirmed the show would return to the original auditions process next year.
He said: "People miss the small rooms, so I think we're going back to the small rooms."
With Adele set to perform her hit single Hello during the show, judge Nick Grimshaw joked: "I hope she doesn't forget the lyrics."
His mentee Che Chesterman performed Hello during the semi-finals last weekend, but managed to get the words wrong.
Chesterman said: "She can show me how it's done."
He added that he planned to greet her with the words "Hello, it's me" - the opening words to her song.
Looking forward to Adele's performance, judge Rita Ora said: "Oh my god I'm so excited! We're going to have the best seat in the house on the panel."
Her mentee, bookies' favourite Louisa Johnson, is currently on voice rest after an illness - but insisted she would be fit to sing by the weekend.
Meanwhile, Cheryl Fernandez-Versini sang the praises of her finalists, Reggie 'N' Bollie.
Looking back on their first performance, she said: "I saw sunshine, light, fun - everything I wanted.
"I saw a vision of what I could do with them from when they first hit the stage, actually. They just needed to be structured."
Ghanaian singer Reggie said: "We feel like being on the show has already changed our lives. All we came here for was the exposure. We had something we wanted to portray, so winning would be a bonus but already we feel like winners."