Star Magazine is reporting that British actor Dominic Cooper is cheating on his girlfriend, American actress Amanda Seyfried. Dominic and Amanda met on the set of Mamma Mia in 2007 (where they played engaged lovers), and Dominic broke up with his girlfriend of 13 years, Joanna Carolan so he and Amanda could date. They’ve been a low-key hot young international couple ever since. You may also know Dominic from his role in The Duchess, where he played the future Prime Minister Charles Grey. You may also know Amanda from her role as one of the Plastics in Mean Girls. Anyway, while Amanda tends to the first round of publicity for Jennifer’s Body in America, Dominic has been in rehearsals for a play in London. And while the cat’s away…

Dominic Cooper has gotten too close for comfort with his Phaedra costar, Irish-Ethiopian beauty Ruth Negga – and his girlfriend Amanda Seyfried is spitting mad over it!

An insider says that while rehearsing at London’s National Theatre, Dominic, 31, and Ruth, 26, were caught making out in a rehearsal room.

“They claimed it was just that: a rehearsal. But they certainly looked shocked when a cast member walked in!”

[From Star Magazine, print edition, Septemeber 7 2009]

But wait! It might not be so scandalous after all. In Amanda’s Allure cover interview, she may have been alluding to difficulties in her relationship with Dominic when she said, “’Things end. Things end. And we’re in a weird business. We’re surrounded by just strange people, strange influences.” British reports are all over the alleged hookup of Dominic and Ruth Negga – sources tell the Daily Mail, “’Ruth has also been talking about Dominic rather fondly in her dressing room to some of the other cast members – which has got people talking that there’s something going on.” Yeah… I think Amanda and Dominic are still technically together, but both know their relationship is going to end soon. You know who she would be cute with? Shia LaBeouf.

Amanda and Dominic are shown out on 4/1/09. Credit: VAH/Fame Pictures