Crown mark reveals royal art

The painting has quadrupled in value after the discovery

A portrait by artist Sir Anthony Van Dyck has been discovered to be a missing piece from the collection of Charles I after restorers found the king's personal cipher.

The painting - Portrait of Mary Villiers, Lady Herbert of Shurland - was officially commissioned by the king in 1636.

The painting, which had been bought by an art dealer last year for £437,500, is now valued at more than £1.6m after the discovery.

The painting was bought by Philip Mould, who runs an art dealership in London.

He said he had had a hunch the painting was more valuable, but could not be sure until it had been restored.

He had the painting cleaned because it was dirty and had been retouched to cover scrapes and chips.

The king's personal cipher found on the back of the canvas

Up to 25% of the original has been retouched in the four centuries since it was painted.

"The canvas was covered, which they often are, to strengthen it. When the restorer took off the back he found the cipher," Mr Mould told BBC News Online.

"We discovered it nine months ago but we've kept it under wraps until the painting was exhibited."

The cipher shows a crown with the letters CR underneath.

Mr Mould said it was "unequivocal" proof of the painting's worth.

The discovery that it was part of the king's collection was especially exciting because Charles I had "the greatest historical collection ever" put together, he added.

"He had treasures from all over Europe. Anything that comes from the collection has an aura attached to it."

He said the painting was also significant because the subject virtually became the king's adopted daughter after her father was murdered in 1636.

Philip Mould says the discovery is very exciting

"And it was commissioned by Charles I from his favourite painter," he said.

The portrait was painted while Lady Villiers lived in Wales and the National Gallery Of Wales was very interested in buying the painting, Mr Mould said.

Collectors and organisation in the U also want to buy the painting but his first responsibility was to find a British home for it, he said.