A portrait of Napoleon Bonaparte sold at auction for £15,000 could be worth more than 100 times that after a deep-clean showed it to be the work of one of France’s most celebrated painters.

The long-lost portrait had been written off as a “dull and lifeless” copy. It was unloved and dirty, with what were believed to be gravy stains on it.

Yet an academic from the University of Reading has studied the painting after it was cleaned and confirmed that it is by the renowned artist Jacques-Louis David, whose works include the famed Marat at his Last Breath.

Dr Simon Lee, from the university’s department of art, insisted the David was genuine and said that although it had yet to be officially valued it could be worth more than £2m. With the Battle of Waterloo bicentenary in 2015, its value could be boosted further.

“People thought it was a copy because it was so dirty,” he said. “It had been unloved and the painting looked dead and flat. The restoration revealed it as a powerful, characterful head.”

Dr Lee performed a series of checks including comparing it to another David portrait at the Frick Collection. Using a conservator’s light, he found that both paintings had remarkably similar mannerisms, modelling and use of drapery.

David painted several portraits of Napoleon, including The Coronation of Napoleon, which currently hangs in the Louvre. The recently unearthed painting shows Napoleon in the uniform of the National Guard in 1813, a time when the British and Prussians threatened to invade France.

The artist was among the foremost European painters from 1780 to his death in 1825 – the most recent sale of his work in 2006 reached £2.1m.

The painting was lost after being hidden ahead of the unexpectedly swift Allied invasion. It was first recorded in a family collection in Scotland before being donated to the Royal Scottish Academy, which sold it to a private collector in New York for £15,000 in 2005.

The academy could not be reached for comment on Sunday.