Adele turned down the chance to perform for Prince William and Kate Middleton at the Royal Wedding last April because a mate had already invited her round for a burnt sausage.
But the Londoner accepted an invitation to belt out romantic tunes for pal Marcus as he walks down the aisle.
She will just have to decide whether to perform wedding classics or her own tracks.
I'd love to hear her version of the Time Warp.
Mumford & Sons frontman Marcus and actress Carey are tying the knot in the West Country just a year after they first got together.
They have kept the details as secret as possible and have invited only close friends and family — plus the biggest pop star in the world.
A source said: "Carey and Marcus are both huge fans of Adele and there will be millions of couples jealous that she is singing for them.
Wedding singer ... Adele will perform at Marcus Mumford and Carey Mulligan's nuptials
"Adele has always wanted to work with Mumford & Sons so when Marcus asked her to play at his wedding she said yes.
"Adele has not been able to perform much at all since her throat op so it's a rare gig.
"It was meant to be a secret but the couple were so excited they've let it slip."
Carey and Marcus first got to know each other as kids, while penpals through their churches, so the ceremony will be religious.
Adele will have to keep her bad language to a minimum.
