Actor Joshua Jackson has opened up about his odd encounter with his estranged father backstage after a play performance in 2005, revealing his dad unexpectedly showed up completely out of the blue after 20 years.

The Dawson's Creek star was left speechless when his dad John appeared in his West End dressing room during the run of his hit play A Life in the Theatre.

Jackson was raised by his casting director mum Fiona after his parents divorced when he was a young boy, and he had no contact with his dad, so he was astonished when John just turned up to say hi.

During an appearance on Canadian talk show George Stroumboulopoulos Tonight, he said, "I was doing a play in London and the stage manager came back and he said, 'There's a guy at the back door, the stage door, who says he's your father.' I said, 'Don't worry about it, he's a crank (fake), I haven't seen my father in 20 years.'

"He comes back and he goes, 'He's about 6 foot 2 (inches), he says your mother's name is Fiona, you were born on June 11, 1978, and you lived in Topanga (California) when you were a kid.' I was like, 'Ok that could be him.'

"And so there he was after 20 years. Just came to, I don't know actually what it was about, other than just to say, 'I'm here.' It was a very odd experience."

And the 34 year old admits it was difficult confronting his mum about the incident: "There's all sorts of loyalty issues that go into that as well because my mother's very much the one who raised us. Just having the conversation (with my dad) felt a bit like I was turning my back (on her), which is not the case; that's my own weird guilt. It's certainly not anything my mother would put on me."