- The 1909 picture showed Prince John posing for a photo with the Queen's father
- The 4.5in by 3.75in photograph is of King George V and Queen Mary's children
- It was signed by them, but it is possible someone had signed John's name for him
- Prince John had epilepsy and autism and died aged 13 in 1919 after huge seizure
A rare photograph of the younger brother of King George VI who was kept out of the public eye because he was mentally disabled has come to light.
The 1909 image shows the 'lost Prince' John posing for the camera with the Queen's father as well as her uncle, the future Edward VIII.
The group portrait of all six children of King George V and Queen Mary was autographed by them, although it is possible someone signed John's name for him.
Prince John had epilepsy and autism and died aged 13 in 1919 from a severe seizure, having spent his final years separated from his siblings at a house at Sandringham.
In the photo, John, then aged four, is sat on Prince Edward's knee, with Princess Mary seated next to them.
Prince Henry is stood on the far left, with Prince Albert in the centre and Prince George on the right.
Henry and George are both in sailors uniforms, with the others dressed in formal clothes.
The 4.5in by 3.75in photo is believed to have once belonged either to a member of the Royal Family, a friend of theirs or a nanny who looked after the young Royals.
It is now being sold by a private collector with Malaga-based International Autograph Auction for £3,000.
Auctioneer Francisco Pinero said: 'This item is a significant because it is extremely difficult to find a signed photo by all six children of King George V.
'At the age of four Prince John was sent to Sandringham House and kept away from the public eye, therefore his autograph is very rare.
'The photo was probably given to a member of the Royal family, someone close to it or perhaps a Royal nanny.'
Attitudes towards mental disability were very different a century ago, with a lack of understanding leading to those with conditions being shunned.
This was typified in a cold-hearted letter by Edward, Prince of Wales, penned shortly after John's death which was uncovered in 2015.
In it, he revealed he considered him only to have been a 'brother in flesh' and 'more of an animal'.
He added the family only ever visited John 'once or twice a year' and his passing was the 'greatest relief imaginable' to them.
The sale takes place on July 22.