The Italian couturier is designing all the outfits for the singer's current world tour, and was reportedly instructed by Aguilera not to include any real animal fur.
But Cavalli admitted to magazine Women's Wear Daily he sent her a stole made from real white fox fur, which she subsequently slipped on to perform gigs in Europe.
Aguilera was tipped off by Peta (People For The Ethical Treatment Of Animals) vice president Dan Mathews, who sent her an email with a link to a video showing foxes getting killed for their fur.
The singer quickly replied, saying she was "very upset" insisting she "only ever wears fake fur". She has now replaced the stole for a fake fur version, in time for gigs in Switzerland, Austria and the Czech Republic this month.