Man left shocked after finding out value of painting following huge mistake made by cleaner

The owner of a valuable painting was given a slightly underwhelming valuation by an art dealer on Antiques Roadshow.

English art dealer Peter Nahum was on hand in a retro 2002 episode of the BBC series to give his opinion on a painting that a guest said has been in the 'family home for 100 years'.

Watch below to see what went down:

The piece of art was drawn up by English artist John Atkinson Grimshaw, a Victorian-era painter from Leeds best known for his nocturnal scenes of urban landscapes.

Famous American painter James Abbott McNeill Whistler, who worked with Grimshaw in his Chelsea studios, once said: "I considered myself the inventor of nocturnes until I saw Grimmy's moonlit pictures."

The episode aired in 2002. (BBC)

Now back to the show, filmed in Knightshayes Court, Nahum was clearly in awe of the painting until he spotted something that would make it only as half as valuable at it should have been.

Due to the unfortunate work of a cleaner in the past, it meant that the valuation was seriously affected.

"Well that's part of the family story," the owner of the painting explains.

"Apparently during the war years, there was a cleaning lady helping in the house and she assumed that the moon was a spot of something that had fallen on the picture and she tried to clean the moon off."

The painting was in the family for almost 100 years. (BBC)

A stunned Nahum then added: "As soon as you explain it then we can see why the moon doesn't balance with the rest of the picture, which it would have done.

"And she's not only had a good scrub here, she's actually removed, I would suggest, a slight tone from over the whole thing.

"So that this circle, the aura of the Moon in the mist, has actually come to prominence.

"Because when an artist finishes a painting, what he does is he balances, with usually pure glazes over the surface, and and in this case this stands out like a sort of archery target.

The art dealer said it was 'relatively minor work'. (BBC)

"It's Grimshaw showing his, feeding the mass market, rather than painstakingly painting the best pictures."

Sadly, the art dealer revealed that the price of the painting would be worth 'more like £15,000'.

He noted that it could have been worth £30,000 if it hadn't been damaged.

If you're posh enough to have cleaners, make sure you explain well in advance that your 100-year-old paintings mustn't be touched.