When the Duke of Sutherland's London residence, Bridgewater House, was bombed in 1941, a vast canvas by the French artist Paul Delaroche was rolled up and taken away to safety at Mertoun, the duke's seat in the Scottish Borders.
There it was forgotten, and the masterpiece by the artist who painted the National Gallery's wildly popular work The Execution of Lady Jane Grey was presumed lost, a casualty of the Luftwaffe. But this year the London gallery's curators were working on a Delaroche exhibition when they were alerted by Michael Clarke, director of the National Gallery of Scotland, to the existence of some unidentified paintings at the duke's home. Curious, they asked to take a look.
So it was that Charles I Insulted by Cromwell's Soldiers (1837), one of Delaroche's masterpieces, was rediscovered. Nicholas Penny, director of the National Gallery, said: "This is huge. Delaroche is one of the greatest 19th-century painters. We think it will create a sensation: it is an extraordinarily powerful work."
The painting – a hefty 2.9 by 2.8 metres – was carefully unrolled at Mertoun in June before being brought to London. The work was stabilised to the extent that it can be displayed from next February, in time for the Delaroche exhibition. It retains shrapnel scars from the bomb, which left a 3m crater in the street outside Bridgewater House, and it is somewhat yellowed by a layer of discoloured varnish.
The exhibition will be the first major show on Delaroche to be held in Britain. The Execution of Lady Jane Grey (1833) is a popular favourite – to the extent that "visitors have worn out the varnish on the floor in front of it", according to Penny – but the artist was until recently rather looked down upon by art historians as sugary, sentimental and stagey. Penny said Delaroche was now ripe for reassessment.
The artist owed much to the theatre and his works, to the modern eye, can even look cinematic. He also drew on 17th-century Dutch realist paintings, and visited England to absorb works by Joshua Reynolds, among others.
The Charles I painting owes a debt to Van Dyck's famous portrayal of the king, and self-consciously recalls the frequently painted scene of Christ mocked by soldiers. In his turn, Delaroche inspired later paintings, not least And When Did You Last See Your Father, by William Frederick Yeames.
Delaroche's favouring of scenes from Tudor and Stuart history above episodes from French history is notable. According to Penny, the artist was in fact exploring the violence and vicissitudes of the French Revolution, but so recent were those events that they were, on the whole, too raw to paint directly.
The Execution of Lady Jane Grey was also sensationally rediscovered, in 1973. Reported to have been lost in a flood at what is now Tate Britain in 1928, curators found it rolled up in storage. When put on display at the National Gallery, it was such a hit that barriers had to be erected in front of it – the first time such measures had been taken at the museum.
The future of Charles I Insulted by Cromwell's Soldiers – whether it will be fully restored and put on long-term public display – is undecided. The duke owns the Bridgewater collection, one of the greatest groups of Old Masters in private hands, which includes works by Poussin, Raphael and Rembrandt.
The collection has been on public view in Edinburgh since 1945, and the National Gallery and the National Galleries of Scotland recently jointly bought one of his Titians, Actaeon and Diana, after a public appeal. According to Penny: "There is no room at his house to hang a picture the size of this. I suppose he could adapt a stable wing or something."