By Jon Boon - Breaking Celebrity News Reporter

When Gerard Butler’s new movie Playing for Keeps is released nationwide next month, it will be a bittersweet experience for the buff Scottish actor.

That's because Butler dated his co-star Jessica Biel but now regrets the way he treated her and considers Jessica the "one that got away."

Gerard, 42, was scared off by Jessica wanting something more serious when they filmed the romantic comedy, so he bolted – a decision he now regrets.

"Gerard doesn’t have many regrets but not treating Jessica with the respect she deserved while they dated is one of them," a source close to the 300 star revealed.

"He genuinely believes she was the one that got away.

"Jessica liked Gerard a lot, she was into his boyish nature, but she wanted something more serious. She was ready to settle down.

"That’s why after getting close on the set of Playing for Keeps back in 2011, their relationship soon fizzled out."

Butler is happy for the newly married Biel because he knows that Justin Timberlake was – and is – the love of her life, "but he still thinks he missed out. He just wasn’t man enough to admit the feelings he had for her at the time," the source divulged.

After her summer fling with Gerard, Jessica rekindled her relationship with Justin, and he popped the question not long after.

They tied the knot in a lavish $6.5M ceremony in southern Italy and gushed about their happiness as newlyweds.