A portrait by famed painted Peter Paul Rubens has been rediscovered, depicting George Villiers, the first Duke of Buckingham and (allegedly) King James I’s lover.

The painting was housed in Pollok House, a National Trust property in Glasgow, where it was long believed to be a copy of a work by the Flemish master that had been missing for nearly 400 years. But Bendor Grosvenor of the BBC’s Britain’s Lost Masterpieces suspected that it could be the real thing and began investigating.

“There was this painting further up by the fireplace and... I thought: ‘My god, that looks like a Rubens,’" Grosvenor told The Guardian. "This picture just seemed to shine out.”

Grosvenor returned over the next two days to inspect the work more closely, but even then, he says “It wasn’t until we peeled it all back that we could be really sure. It’s one thing for an optimist like me to have a hunch, but quite another to prove it.”

An episode of the series airing this week chronicles how overpainting and centuries of grim obscured Rubens' trademark brushwork. After art restorer Simon Gillespie returned the painting to its original splendor, it was authenticated as a Rubens by the Rubenshuis museum in Antwerp.

“[It is] rare addition to Rubens’s portrait oeuvre, showing how he approached the genre," said Rubenshuis director Ben van Beneden.

The portrait is actually study for a larger painting that has since been destroyed. “It’s just a sketch, but in the 18th and 19th century, unfinished work wasn’t as appreciated as it is today, so you get a lot of pictures which are finished off by later artists," Grosvenor explains. "So this was tidied up by someone and, as a result, it began to look very stiff and more like a copy."

The Duke of Buckingham’s relationship with James I has long been debated: Though married to Anne of Denmark, James was known to lavish attention on young courtiers, of which Villiers was the last. (James' nickname for Villiers was "Steenie," after the angel-faced St. Stephen.)

The two gushed over each other in letters—James called Villiers his "wife" and the Earl told his king "I will live and die a lover of you." In 1617, King James told the Privy Council in 1617, "You may be sure that I love the Earl of Buckingham more than anyone else—and more than you who are here assembled."

Villiers was assassinated at the age of 35 in 1628, three years after James died.

“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," says Grosvenor.

The restored Rubens will go on display at Glasgow's Kelvingrove Museum on Thursday.