He saw the apparition while performing Waiting for Godot with Sir Ian McKellen.

Stage hands believe he saw the ghost of John Baldwin Buckstone, who was actor-manager of the Theatre Royal Haymarket in the mid 19th century and a friend of Charles Dickens.  

Upon coming offstage for the interval, Stewart told his co-star that he saw a man standing in the wings wearing what looked like a beige coat and twill trousers.

Sir Ian asked him: "What happened, what threw you?"

"I just saw a ghost. On stage, during Act One," Stewart replied.

The episode was related in a documentary about the Theatre Royal Haymarket, produced by television channel Sky Arts.

However, it appears cameramen failed to capture images of the ghost itself.

Buckstone had a long association with the Theatre Royal, first as a comic actor, then as a playwright and finally as its actor-manager from 1853 to 1877, during which time it put on some 200 productions. The house became the leading comic theatre of the day.

He did not die in the building, passing away peacefully at home in Sydenham, Kent, after a long illness in 1879 aged 77. But theatre lore professes that he nevertheless haunts the place to the present day.

Nigel Everett, a director of the theatre, said: "Patrick told us all about it. He was stunned. I would not say frightened, but I would say impressed."

Appearances of Buckstone were not that frequent, Mr Everett said, with the last being by a stage hand about three or four years ago.

He added: "The last time an actor saw him would have been I think Fiona Fullerton, playing in an Oscar Wilde, 10 or 12 years ago.

"The ghost tends to appear when a comedy is playing."

While he said he did not consider Waiting for Godot to be a comedy, he thought their production did have comic aspects.

"I think Buckstone appears when he appreciates things," he added. "We view it as a positive thing."