From street vendors peddling food to singers performing to a crowd, a 17th-century Dutch painting in the Royal Collection captures all the rustic charm of a village fair. But work undertaken by Royal Collection Trust conservators ahead of a new exhibition opening at The Queen's Gallery, Buckingham Palace next month has revealed that all was not quite as it seemed. Painstaking cleaning of the painting has uncovered a squatting figure relieving himself in the foreground, hidden for more than 100 years under overpainted shrubbery.

Painted in 1643, A Village Fair with a Church Behind by Isack van Ostade is one of 27 works going on display in the exhibition Masters of the Everyday: Dutch Artists in the Age of Vermeer. It was acquired in 1810 by George IV, when Prince of Wales, and hung in the Middle Room at Carlton House, the Prince's London residence on Pall Mall. Inventories of Carlton House in the Royal Archives show that the coarse, comic depictions of peasant life in A Village Fair with a Church Behind would have been entirely to the future king's taste.

It is believed that the offending figure was painted over in 1903, when the work, which by then hung in the Picture Gallery at Buckingham Palace, was sent for treatment by an art restorer. The modified painting, perhaps now more in tune with Edwardian sensibilities, was returned to the Picture Gallery, where it hung for several more years. A similar alteration had been made to A Village Revel by Jan Steen, 1673, also acquired by George IV and in the Royal Collection. The painting shows a group of country people drinking and brawling outside an inn, symbolising human folly. Conservation revealed that the tavern sign was originally painted with an image of a man with his buttocks exposed, which at some point had been overpainted with a bull's head.

Desmond Shawe-Taylor, Surveyor of The Queen's Pictures and curator of the exhibition said:

'Dutch artists often include people or animals answering the call of nature partly as a joke and partly to remind viewers of that crucial word 'nature', the inspiration for their art. Queen Victoria thought the Dutch pictures in her collection were painted in a 'low style'; two years after her death perhaps a royal advisor felt similarly'.