The 'Collide' star had been expected to perform, but her appearances were left in doubt following a delay in the release of her third studio album Glassheart.
However, speaking exclusively to Digital Spy at the Hopefield Animal Sanctuary Christmas Fete, Lewis revealed that her performances will take place to promote the release of a three-track EP in December.
"I'm going to be on X Factor UK and US. I should be doing the final," the 26-year-old said, adding that continued work on her new collection was going "really good".
Lewis continued: "I'm really glad to be back because one of the guys I'm working with on the album is here. I'm going to be working with Fraser [T Smith] for a little while. I've just done the EP, so that's all recorded and will be out this year."
Lewis also revealed that she will sing at this year's Royal Variety Performance, marking her second time performing at the annual event. She said that she is still deciding if the winner of her recent dress competition will have their design worn during the Royal Variety or X Factor appearance.
Discussing the Christmas Fete at the animal sanctuary - of which she is a patron and trustee - Lewis said that they had almost hit their £100,000 target for Christmas.
"The next event is in December, which is the Christmas Carol concert. We've got some other people singing, which I'm really excited about," she teased. "There's someone else special who I really hope can make it... but we'll see!
"Another fundraiser in February is going to be our Valentine Gala. Next year we definitely want to raise as much as we have this year - and top it!"
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