star Dominic Cooper
pledged to marry and have children with his childhood sweetheart in a bid to win her back - whilst allegedly enjoying a fling with his co-star Amanda Seyfried
The British actor and Seyfried, who play lovers in the Abba-inspired movie musical, began their on-set affair at a time when Cooper had temporarily split from Joanna Carolan, his girlfriend of 12 years.
She tells U.K. magazine Grazia, "He didn't want to (be on a break) but I insisted. He went off to Greece and I knew about Amanda straight away. Dom didn't tell me - I heard it from friends. I honestly thought it was a five-minute fling.
"During this time I even went to the Mamma Mia! premiere with Dominic. Of course Amanda was there and it was desperately awkward. They weren't supposed to be seeing each other then but in my heart I knew that they were, and those were the hardest days."
After an emotional heart-to-heart, the pair decided to start afresh and vowed to settle down with each other: "After the initial confusion, I suddenly realised I did want to be with Dominic for the rest of my life...
"I said I wanted to marry and have kids and he said that's what he wanted, too. I completely believed him."
But Carolan was left devastated to discover Cooper was still seeing the Mean Girls
star behind her back.
She adds, "I took it for granted he'd end his relationship with Amanda. He'd say he had and then I'd hear through friends that he hadn't.
"He was still always flying off to L.A. where she lives and I know Dom - he's someone who needs a bosom as a pillow. By then it did feel like betrayal. I realised I was becoming a victim and I refused to be that woman."
And although Carolan has kept in touch with Cooper, she struggled to cope with her heartache following their split: "I've cried a lot of tears and have woken up to the reality that the future I wanted is never going to happen.
"It was reported that we are back together after he had promised to end his affair. That's not true. We still love each other, but we're not together. I also care about Amanda: she's young and vulnerable and has been upset by what's been said."