British actor Idris Elba has opened up about the "tragic" moment he realised his ex-girlfriend's child was not his own, insisting it was like taking "a punch in the face".

The Thor star was thrilled when he became a father for the second time in 2010 after his partner delivered a baby boy, and he gushed about the magical moment of having a son online - but his joy was short-lived.

He developed suspicions about the paternity of the baby and underwent a Dna test, which determined he was not the kid's dad.

He tells GQ magazine, "It wasn't immediately obvious - well, it was, because he didn't look like me. But it wasn't immediately obvious what had gone down...

"To be given that (news that he was the boy's dad) and then have it taken away so harshly was like taking a full-on punch in the face: Pow. You know, the truth is, like, even admitting it, I'll probably get laughed at for the rest of my life. But it is just tragic, and it happened.

"But I wasn't knocked out. I stood right the f**k back up, and I ain't (sic) aiming to take another punch in the face ever again. Do you understand what I'm saying? It happened to me. I moved on."

The 41 year old has a daughter, named Isan, with his ex-wife, Kim.