An "exceedingly rare" photograph signed by two members of the royal family has been found in a Cardiff bric-a-brac store.
The image shows the Queen and Prince Charles with members of the Privy Council after a meeting at Buckingham Palace in 1981.
Both royals have signed it.
Auctioneers said it was bought by a member of the public for "about £20" but is expected to fetch up to £1,000 at auction.
The meeting on 27 March 1981 marked the Queen giving Prince Charles consent to marry Princess Diana, who is not pictured, under the Royal Marriages Act.
It will go under the hammer with Dominic Winter Auctioneers Ltd, of South Cerney, Gloucestershire, next month.
Chris Albury, auctioneer and senior valuer, said it was the first time he had seen something signed by more than one member of the royal family.
"A guy saw it in the window of the shop and wasn't entirely sure if the signatures were authentic," he said.
"I did a double take when I saw it, I thought I better not get too excited until I check it out properly."
Mr Albury initially thought the signatures might have been autopen - ink signatures made by a "robot", which the royals use to sign Christmas cards - but further tests showed they were "genuine".
"What I like about it is that, when it comes to royal autograph collecting, the key is to collect the monarchs, it is the gold standard," he added.
"With Prince Charles next in line, it could become an item that has not one, but two, monarch signatures.
"Whether it's a one-off, I don't yet know. The next part of my research is to find out exactly why they both signed it."
The frame of the photograph has also been signed by George Thomas, 1st Viscount Tonypandy, who is pictured second on the right in the front row.
It will go under the hammer on 5 April.