In an interview with the Sunday Times Magazine, Gambon, 74, revealed that learning scripts takes him longer and that he quickly forgets them.
"It’s a horrible thing to admit, but I can’t do it. It breaks my heart. It’s when the script’s in front of me and it takes forever to learn. It’s frightening,” he said.
The actor, who played Dumbledore in six Harry Potter films, admitted that he had recently resorted to using ear pieces to feed him lines.
But his forgetfulness had become so pronounced that he realised six months ago it was time to bring down the curtain on a five-decade career in the theatre.
"There was a girl in the wings, and I had a plug in my ear so she could read me the lines," he said. "And after about an hour, I thought, ‘This can’t work.’ You can’t be in theatre, free on stage shouting and screaming and running around, with someone reading you your lines."
Gambon put his forgetfulness down to his age and worried that it was a sign of Alzheimer’s, but his doctors have given him the all-clear.
This isn’t the first occasion Gambon has struggled with his memory. In 2009, he was taken to the hospital twice with panic attacks brought on by the stress of forgetting his lines during rehearsals at the National Theatre in London.
Though he may never grace the stage again, Gambon will continue with this film and TV career where the stress of remembering is significantly less and he can be fed his lines.
Gambon will next appear in the BBC adaptation of J.K. Rowling’s The Casual Vacancy as well in the Dad’s Army big-screen remake.