Justin Bieber's special guest on his Christmas album will be Mariah Carey.

The 'Baby' singer has an all star line up for his 'Under the Mistletoe' festive record, including Busta Rhymes, Boyz II Men, and Usher, but was keeping one extra special star a secret - even from the album's producer - until earlier today.

He tweeted: "oh yeah... didn't I promise everyone I would tell them who my last special guest is on my Christmas album #UnderTheMistletoe ???

"I mean we know we got my big bro @UsherRaymondIV and@thebandperry and @BusaBusss and @BoyzIIMen on the album - but who is the last guest??? (sic)

"Who is the living legend that I got on the album???? I mean the fact that I might be singing with one of the greatest voices ever one of the greatest songs of Christmas ever.(sic)"

Justin then tweeted a link to a video of Maria in which she says: "What's up I'm Mariah Carey and I'm here today to tell you a little secret. Justin and I are doing this duet together, it is my song 'All I Want For Christmas', which I wrote and produced.

"I'm very excited about it. I'm sure many people are surprised and you're going to be more surprised when you hear it. The song sounds great and I'm very excited for you all to hear it this Christmas and be totally festive with our festive collaboration."

Justin's first single from the album, 'Mistletoe' will be released on October 18 and the album will be released on November 1.

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