At least 20 unpublished stories by Anthony Burgess, the author of A Clockwork Orange, have been discovered by researchers sorting through his papers at a research centre in Manchester, the city in which he was born.

The short stories, unproduced film and theatre scripts and hundreds of musical compositions have emerged from the contents of three houses in London, Monaco and Italy, bequeathed to the International Anthony Burgess Foundation after the death of his widow, Liana, four years ago. Burgess died in 1993.

Among the archive are 50,000 books and 20,000 photographs, symphonies, poems and unfinished or rejected scripts for television and film projects, including lives of Atilla the Hun, Sigmund Freud and Michelangelo and a play about Harry Houdini that he collaborated on with Orson Welles, another frustrated creator of unproduced projects.

Will Carr, the deputy director of the research centre, said: "We are discovering things all the time. There is a lot of stuff, and we are still unpacking cardboard boxes.

"He was a good short fiction writer and, particularly early in his career, he would write these things and then put them away and forget about them. They have never been read or published. The stories are very good, very funny and pungent. You can see how his writing developed."

Burgess published 33 novels in a prolific career and was also a critic, broadcaster, scriptwriter and composer.

Among the papers is the first completed music he wrote, his original screenplay for A Clockwork Orange – rejected by the film's director, Stanley Kubrick, who eventually wrote the script himself – an unpublished history of London, a ballet score about the life of Shakespeare and a musical about Leon Trotsky. There is even a script for another, unmade, Kubrick movie, which would have been about Napoleon. One of the discovered compositions, A Manchester Overture, is being played by the BBC Philharmonic Orchestra, and the foundation intends to publish a collection of the unknown short stories next year.

Andrew Biswell, the foundation's director and a Burgess biographer, told the BBC: "A lot of the stories are very nasty and tending towards the supernatural – a lot of ghost stories or stories about gods who come down to earth. The amount of material which people don't know about I think heavily outweighs the known. Even though Burgess was productive and he published a lot, a good deal of what we've got here has always been below the waterline. It has never been made available in a public way until now.

"I'm staggered by the extent of the collection sometimes. I come down into the basement and I look at it and I think, my God, did this man never sleep?"

Carr said: "Burgess has been forgotten about in Manchester, but people are excited to come here and find out about him.

"I think it happens a lot to writers – they dip out of favour after their death and then get rediscovered and take their place. It is the 50th anniversary of the publication of A Clockwork Orange next year, and we are expecting a resurgence of interest then."