IT ADORNED the dining room of a stately Glaswegian home for decades – a portrait of a Duke which was hung, unheralded, and described as a mere copy.

Now the Duke turns out to be worth a King’s ransom as experts confirm it is a masterpiece by one of Europe’s great painters, Peter Paul Rubens, and an artwork thought lost for 400 years.

The portrait of George Villiers, the Duke of Buckingham has been in Glasgow’s art collection for years, ever since Pollok House and its contents, collected by Sir William Stirling-Maxwell, was gifted to the city in 1966 by Dame Anne Maxwell Macdonald.

The painting previously hung in Pollok House with the description ‘after Rubens’ – it was thought to be a copy of the original.

Now it has been dated to 1625, formally attributed to the Flemish master, and is worth – insiders at the museums believe – “tens of millions” of pounds.

In 2016, a Rubens painting Lot and His Daughters from 1613, sold at Christie’s auction house for £44.8m. The Pollok House painting’s price on the open market is moot, however, as Glasgow Museums will not be selling.

The true identity of the painting was discovered via the BBC show, Britain’s Lost Masterpieces, and joins the list of art treasures owned by the city, a collection of 1.2m works insured for £1.4billion.

The painting will go on show at the the Kelvingrove Art Gallery and Museum from September 28.

It was identified by art historian Dr Bendor Grosvenor whilst working with presenter and historian Emma Dabiri for a new series of the BBC4 show.

Councillor David McDonald, chair of Glasgow Life, which runs the city’s museums and galleries, said: “Unsurprisingly we are beyond delighted to discover the painting is by Rubens, an artist renowned globally as one of the most important painters in history.

“George Villiers, First Duke of Buckingham is sure to become one of the undoubted highlights of any visit to Kelvingrove.”

Conservation work has been carried out on the painter by Simon Gillespie, a leading restorer.

He removed layers of dirt and paint that had concealed many of Rubens’ trademark techniques, which had created doubt over the paintings true attribution.

Once cleaned, the portrait underwent reassessment and the revised attribution to Rubens was confirmed by the director of the Rubenshuis Museum in Antwerp, Ben van Beneden.

Dr Bendor Grosvenor said: “The chance to discover a portrait of such a pivotal figure in British history by one of the greatest artists who ever lived has been thrillingly exciting.”

The BBC programme will detail “overwhelming evidence” of its new attribution, including a technical analysis of the panel on which the portrait was painted proved that it was prepared in the manner used in Rubens’ studio.

Dendrochronology – examining the tree rings of wood to date it – showed that the panel was likely created in the early 1620s, and a number of alterations revealed by cleaning and X-ray analysis in areas such as the hair and costume, demonstrate that the painting “could not be” a copy.

Karen Cornfield, Property Manager for Pollok House said: “This is such exciting news for Glasgow and shows what a great eye Sir William Stirling Maxwell had.

“Pollok House is packed with beautiful paintings that he collected over the years.

“We hope the programme will encourage visitors to come to Pollok, explore the collection and to learn more about the family who brought so many great artworks to Glasgow.” This episode of Britain’s Lost Masterpieces will broadcast on Wednesday 27th September on BBC Four at 9pm.

Pollok House has been operated by the National Trust for Scotland since the late 1990s.