Harry Potter actor Daniel Radcliffe called Alan Rickman "undoubtedly one of the greatest actors I will ever work with" as he paid tribute to his co-star, who has died aged 69.

Rickman played Radcliffe's Hogwarts' nemesis Professor Severus Snape in the franchise, but Radcliffe said he was the opposite in real-life to his formidable on-screen persona.

"People create perceptions of actors based on the parts they played so it might surprise some people to learn that contrary to some of the sterner (or downright scary) characters he played, Alan was extremely kind, generous, self-deprecating and funny. And certain things obviously became even funnier when delivered in his unmistakable double-bass," he wrote on his Google plus page.

The 26-year-old also recalled how supportive of his career Rickman had remained, even after they stopped being co-stars.

"He is also one of the loyalest and most supportive people I've ever met in the film industry. He was so encouraging of me both on set and in the years post-Potter. I'm pretty sure he came and saw everything I ever did on stage both in London and New York. He didn't have to do that.

"I know other people who've been friends with him for much much longer than I have and they all say, 'If you call Alan, it doesn't matter where in the world he is or how busy he is with what he's doing, he'll get back to you within a day'," he wrote.

The two played opposite each other in the eight film adaptations of JK Rowling's books, with Radcliffe cast in the role as The Boy Who Lived, at the age of 11, and he wrote his appreciation that Rickman treated him as an actor, not a child, on set.

He wrote: "As an actor he was one of the first of the adults on Potter to treat me like a peer rather than a child. Working with him at such a formative age was incredibly important and I will carry the lessons he taught me for the rest of my life and career."

He concluded: "Film sets and theatre stages are all far poorer for the loss of this great actor and man."