Joseph Wright original work uncovered at Derby Museum

Conservator Mark Roberts said the work may have been painted over to cover up a mistake by a restorer

A painting that languished in a museum store room for more than 50 years dismissed as a replica could be worth up to £1m.

Experts say the painting at Derby Museum is an original by Joseph Wright, one of the UK's most acclaimed artists.

Coliseum by Moonlight was painted in the 1780s after Wright toured Italy and will now undergo a year's restoration.

Museum staff said it was placed in storage after being badly over-painted by a restorer in the early 1960s.

Lucy Bamford, senior curator of art, said subsequent staff did not know about the restoration and the painting was mistakenly re-catalogued as being by a follower of Wright.

'Absolutely terrible'

It was finally uncovered after the museum's conservator, who was working on its companion - Coliseum by Daylight - discovered literature that showed the museum was meant to hold two paintings of the Coliseum by Wright.

The painting will be on show at the museum in the summer as part of the Grand Tour art event in the East Midlands

"We said 'That can't be. There's one in the store but it's absolutely terrible'," Ms Bamford said.

"Once we got it out... we realised that there was something behind that painting which was worth investigating."

Infrared technology will be used to reveal what is left of the painting and conservator Mark Roberts will use solvents to remove the over paint.

Joseph Wright 1734-1797

• Known as Joseph Wright of Derby, he was the first major English painter to be based outside London

• Wright was the first artist to depict industry and scientific experiments of the age

• He was noted for his use of chiaroscuro, which emphasises the dramatic contrast of light and dark

• One of his most famous works is An Experiment On A Bird In The Air Pump, from 1768), which recreates an experiment by the 17th Century English scientist Robert Boyle

Source: BBC Your Paintings

Ms Bamford said Wright was "undoubtedly one of Britain's greatest painters".

She said placing the two paintings together would show how Wright was thinking about comparing and contrasting different lights sources, effects and compositions.

"It's a really interesting insight into Wright's working methods and the way that he thought," she said.

Wright was born in Derby and his patrons included Wedgewood and Arkwright.

He became associated with the Industrial Revolution of the 18th Century but his main interest was the rendering of light.

The painting will be displayed mid-way through its restoration process at the museum in the summer as part of the Grand Tour art event in the East Midlands.

The conservator was working on Coliseum by Daylight (above) when he discovered literature that showed the museum was meant to hold two paintings of the Coliseum by Wright