Artwork is rare survivor by colour print inventor

The 1721-22 print is based on a portrait of Charles I's eldest children - (from left to right) the future Charles II, James II and Mary, Princess of Orange

A work of art hidden in a dark corner of a National Trust manor house for 150 years has been revealed as a rare surviving print by the inventor of colour printing.

The copy of a portrait by Dutch artist Sir Anthony van Dyke was assumed to be oil-on-paper until curators at Oxburgh Hall, Norfolk, sent it away for conservation.

It was discovered the reproduction of the Three Eldest Children of Charles I was in fact a print by Jacob Christoff Le Blon, who first created the three-colour printing process.

"Only three Le Blon prints of it are known to survive, so to have discovered a fourth is really exciting," said National Trust curator Jane Eade.

It has hung in the same place beneath the staircase of Oxburgh Hall since the late 19th Century

The discovery at the home of the Bedingfield family, who were devout Catholics, suggested they were secret Jacobites - supporters of the Catholic Stuart royal family in exile instead of George I.

The Protestant king inherited the British throne in 1714.

The portrait includes the future James II, the last Catholic monarch of Britain, whose son James Francis Edward Stuart attempted to take back the throne in a failed uprising in 1715.

"Copies of this painting are known to have been popular with those sympathetic to the Jacobite cause, and it would have been quite feasible that the print has spent its whole life at Oxburgh - although we lack enough evidence to prove it," said Oxburgh collections and house manager, Ilana van Dort.

Heavy varnish applied in the 19th Century led curators to believe it was an oil-on-paper reproduction, rather than a rare hand-coloured print by Jacob Christoff Le Blon

The original portrait is part of the Royal Collection, to which Le Blon was given access by George I, and his print dates to 1721-22.

He was the first to create a three-colour printing process – the forerunner of the CMYK colour printing used today.

The revolutionary method used mezzotint, a monochrome printmaking process, with separate plates inked in blue, yellow and red, and superimposed on one another in order to create variable depth of hue.

Until Le Blon, artists had inked colours one beside the other on a single printing plate.

The Bedingfields have lived at Oxburgh Hall since 1482 and were devout Roman Catholics for centuries - the hall even has an Elizabethan-era priest hole

A specialist paper conservator at the trust's Royal Oak Conservation Studio at Knole, Kent, carried out conservation treatment.

Analysis helped identify colours Le Blon was known to have used such as indigo, carmine or red lake.

"A thick layer of 19th Century varnish was particularly challenging," said Ms Eade.

The story of its conservation will feature in the new BBC Two series of Hidden Treasures of the National Trust, which starts on Friday.

This print is only the fourth known to survive into the 21st Century, said the National Trust