A painting given a reserve of £1,500 sold for more than £2m at auction after bidders became convinced it was a Rembrandt self portrait.
People are divided on whether this is a Rembrandt self-portrait
The work had hung on the wall of a house in Cirencester for several years before being sold in the town.
Philip Allwood - from Moore, Allen and Innocent - said he thought the portrait might be a Rembrandt but its owner said it had been checked and was not.
But bidders who drove the price up to £2m were convinced otherwise, he added.
Mr Allwood said he had to make sure the painting was not misdescribed in the auction room's catalogue.
"When I first saw it I said it looked very much like a Rembrandt and was assured by the client it had been checked out years ago and it wasn't," he said.
Deciding to do some more research on the painting, Mr Allwood spoke to the Rijksmuseum in Amsterdam.
Experts there assured him that, while it was of the period of Rembrandt, it was "probably not" painted by him.
He said the auction house was not sure so was very careful not to misdescribe the painting in the catalogue.
"But on the day of the auction both the winner and under bidder seemed convinced it was genuine," Mr Allwood added.
"The buyer who wishes to remain anonymous seemed very relaxed spending that sort of money."