It is often said that modern art is rubbish, but never did it ring as true as when an art gallery cleaner binned a work by Damien Hirst because he thought the installation was exactly that - leftover rubbish.

Emmanuel Asare thought the piles of full ashtrays, half-filled coffee cups, empty beer bottles and newspapers strewn across the gallery were the remnants of a party in the west London gallery.

Although that is what it was, this rubbish had been arranged by Hirst into an impromptu installation, which increased its value by thousands.

Hirst had arrived at the launch party of a new exhibition of his work, Painting-By-Numbers, on Tuesday night at the Eyestorm gallery. When asked how he would like his prints to be displayed in the windows, the artist, famed for his formaldehyde animals, decided to create a new work.

The next morning when Mr Asare arrived for work, he decided to clean up the mess straight away by putting it all in bin bags. Mr Asare said: "As soon as I clapped eyes on it I sighed because there was so much mess.

"I didn't think for a second that it was a work of art - it didn't look much like art to me. So I cleared it all into binbags and dumped it."

Staff were dispatched to find the binbags in the rubbish, and salvaged the various objects, which they used to reconstruct the installation from photographs taken earlier.

Hirst, 35, said the piles of junk represented an artist's studio and said the mistake was "fantastic. Very funny."

Charles Thomson, co-founder of the Stuckist art movement, which favours the traditional skills of drawing and painting, praised Mr Asare's action.

"The cleaner obviously ought to be promoted to an art critic of a national newspaper. He clearly has a fine critical eye and can spot rubbish, just as the child could see that the emperor wasn't wearing any new clothes," he said.