A famous image of Bonnie Prince Charlie - widely regarded as the best likeness ever found of the Jacobite hero - is actually a picture of his brother, it was claimed on Tuesday.

The National Portrait Gallery in Edinburgh has conceded that the portrait, purchased in 1994 at a cost of £22,000, is probably Prince Henry Benedict.

Charles, the Young Pretender, was the elder son of the Old Pretender, James Edward Stuart, and the grandson of James II and VII. He fled into exile after his forces were defeated at Culloden.

His younger brother, Prince Henry Benedict, was known as the Duke of York or Cardinal York until his brother's death in 1788, when he claimed the title of Henry IX.

Art experts say re-identification of old portraits is not uncommon, and does not diminish the significance of the five-foot tall pastel painting, created by French artist Maurice Quentin de La Tour.

The painting is currently in storage while the portrait gallery undergoes renovation. But when it reopens in 2011, the image will be renamed.

Stuart portrait expert Dr Edward Corp said the painting is an almost exact likeness of another portrait of Henry.

He said: "The weight of evidence - perhaps regrettably - supports the argument that the pastel in the Scottish National Portrait Gallery shows Prince Henry rather than Prince Charles.

"Whether the portrait does or does not show Prince Charles really does matter. It is not merely the catalogue of the Scottish National Portrait Gallery which needs to be corrected.

"The impression which an entire nation has derived of this important historical figure should also be changed."

A spokesman for the National Galleries of Scotland said: "Dr Edward Corp, an authority on Jacobite imagery, has defended the identification of the sitter in our pastel as Charles.

"However, in a recent article for the British Art Journal, Dr Corp re-considers this position.

"In the light of this development, we have reassessed our own position and, on balance, accept that it is more likely that our pastel depicts Prince Henry Benedict, and not his elder brother."

British Art Journal editor Robin Simon said: "There is no doubt at all. But it's rare that anything is so spectacularly wrong."