The manager asked the audience to leave the theatre, and the star of hit BBC sitcom was treated on stage.
According to the Telegraph, the audience at the Trafalgar Studios in Whitehall, central London, initially thought that the collapse was part of his act, but they later came to know of the gravity of the situation when Horne?s co-star Simon Paisley Day called out: "I think something has gone wrong. Is there a doctor in the house??
The 30-year-old actor was then taken to the hospital, but his condition is still unknown.