Idris Elba was devastated when he discovered his ex-girlfriend's child wasn't his.

The 'Luther' actor admits the moment a paternity test confirmed his US-based ex - whom he dated for several years - had been lying to him about fathering the son he had always longed for was the most ''tragic'' time of his life.

He told GQ magazine: ''It wasn't immediately obvious - well, it was, because he didn't look anything like me. But it wasn't immediately obvious what had gone down. The celebration of having a son - from a man's perspective, it's massive.

''To be given that and then have it taken away so harshly was like taking a full-on punch in the face. Pow!''

The British hunk isn't embarrassed about admitted he was duped, but has now moved on and won't make the same mistake again.

Idris said: ''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 aiming to take another punch in the face ever again ... It happened to me. I moved on.''

The 41-year-old star believes the heart-wrenching incident was a life lesson from karma because of his bad boy past.

Idris added: ''I've not been an angel in my life, either - do you know what I'm saying? To a certain extent, what goes around comes around. But for me in the future, I'm about being comfortable. That's it.

''I was running with cats. I mean, I was DJing, but I was also pushing bags of weed; I was doing my work. I had to. I know that sounds corny, but this is the truth.''