Rembrandt self-portrait that sat in a Cotswolds house for 50 years worth up to £20m

It had a guide price of £1,500, but sold for £2million.

Now a tiny self-portrait of Rembrandt, that at first was thought to be worth relatively little because of uncertainties over who painted it, could be worth ten times as much again.

The value rocketed after experts confirmed that the painting is indeed a self-portrait by the Dutch master Rembrandt.

The 17th century work had been in a house in the Cotswolds for 50 years.

It was finally sold in October last year.

Although it was entitled The Young Rembrandt as Democrates, the seller could not prove to the auction house that it was by the artist's hand.

Several bidders took a gamble, however, and pushed the price much higher.

Their instincts have been proved right.

William Noortman, from Noortman Master Paintings, which specialises in Dutch and Flemish masters, said: 'That picture's clearly worth between £15million and £20million.'

Philip Allwood, head auctioneer at the Gloucestershire firm that sold the piece, Moore, Allen and Innocent,said: 'On the basis that it was not a Rembrandt, we made a very cagey estimate in the catalogue.

'How wrong could we be?'

The anonymous buyer has loaned the 9.5in x 6.5in painting to the Rembrandt House Museum in Amsterdam.