Charlie Sheen's porn star ex Bree Olson has joined director Tom Six and Julia Roberts' brother Eric in the third Human Centipede movie.

Writer/director Six will actually appear in the third instalment, Final Sequence, which will conclude the trilogy about a sick doctor who drugs victims and then sews them together, linked mouth-to-anus.

Roberts tells the third film is "really horrible", adding, "We have a centipede that is made of prison inmates, and they're all hooked together. When you see this, you will never want to commit a crime and go to prison."

Olson and former wrestler Tommy 'Tiny' Lister will have supporting roles in the film.