Another Hollywood couple in a cheating scandal?

Reports surfaced this weekend that Liev Schreiber, 44, has cheated on longtime partner (and mother of his two sons), Naomi Watts, 43 -- and that it happened the same weekend Ashton Kutcher allegedly cheated on Demi Moore.

PhoebeGraceLievSchreiber230.jpgWas something in the water??

According to, Liev had been filming Mental in Australia (where Naomi Watts is from.) In the film, a swimwear model named Phoebe Roberts (left, in Kulu Swimwear) served as body double for Liev's co-star, Toni Collette.

Schreiber and Roberts supposedly hooked up after the movie's wrap party on Sept. 24. The actor allegedly took Roberts back to his apartment.

Liev's rep has denied the allegations, saying, "This is a complete fabrication."

Schreiber and Watts have been a couple since 2005. We spotted them out together as recently as Sept. 27 in NYC -- only a few days after the alleged hook-up with Roberts. See the pic after the jump.