For as long as anyone can remember, it was an unremarkable portrait hanging on the staircase of a Victorian hotel in Cheshire. Guests brushed past it without a second glance.

The painting has now been removed to a local museum after the artist was identified as Gainsborough (1727-88), who is revered for portraits that capture the elegance of his age with a technical brilliance.

Hugh Belsey, one of Britain’s foremost Gainsborough specialists, spoke of his astonishment at seeing the unknown work, which he said was likely to be worth hundreds of thousands of pounds. “It is always exciting to come across a portrait that has remained unknown for centuries,” he said.

“It shows all the characteristic sensitivity of Gainsborough’s late work. It was painted at immense speed and shows all the subtleties of the artist’s deft brush. It has an honest straightfor-wardness that only Gainsborough could bring to portraiture.”

The canvas, measuring 36 x 28 inches, depicts James Tomkinson, a Cheshire solicitor who commissioned two portraits from Gainsborough - this and one of his grandsons, which is in a museum in America. The newly discovered picture dates from about 1784. Gainsborough had no connections with Cheshire and would have painted it in London.

Mr Belsey declined to reveal the name of the hotel. Instead, he gave The Times a statement from it: “It is fantastic to know that a painting which had hung in our restaurant [before being moved upstairs] has been brought to a wider audience.”

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010