The scene is one of the most treasuredA poll of gallery-goers in 2005 placed it second only to Turner's Fighting Temeraire. in all of British art. John ConstableThe parts of Essex and Suffolk which he painted are now known as Constable Country.'s painting The Hay Wain shows a landscape which seems quintessentiallyMost typically. English, as a farmer drives a wagon across the River Stour in Essex. Though it initially failed to find a buyer, it came to be seen as revolutionary, with Constable hailed as the greatest landscape painter of the age.

But a shadow has fallen across this and other masterpieces in the National Gallery. This week the gallery published its research into art works acquired between 1824 and 1880, exploring whether anyone involved with them had a connection with slavery. And some of the discoveries made for uncomfortable reading.

The researchers found at least 67 people, from painters and their subjects, to donors, who were linked to the trade - though to confuse the debate, 17 of them were also involved in its abolition.

Most controversial of all were the revelations about John Julius Angerstein, from whom the British government bought 38 Old Masters - including RaphaelAn Italian painter who was one of the greatest Renaissance artists.'s brilliant portrait of Pope Julius II - as the core of the new gallery's collection in 1824. We now know that Angerstein made a fortune from insuring slave-transporting ships.

Another masterpiece, BotticelliA 15th-Century Florentine artist best known for his The Birth of Venus and Primavera.'s Mystic Nativity, once belonged to William Ottley, who owned 17 slaves in Antigua.

Reactions have been mixed. One expert on African history, Professor Hakim Adi, complained that the gallery had not gone far enough: "Acknowledgment is a very fine thing, but it is not a reparation for that crime." The Daily Mail's John Abiona insisted that great paintings were being "tarnished" on "very tenuous" grounds.

The Hay Wain, for instance, was once owned by Edmund Higginson, who inherited money from an uncle who traded goods made by slaves. But it also appears to have been owned by at least four other people with no known connection to slavery.

The National Gallery is not alone in wondering how to deal with controversial works. Many other galleries will be watching its next move. It could, for instance, follow Amsterdam's Rijksmuseum in exhibiting paintings beside evidence of slaves' suffering.

Debate has always surrounded the relationship between a work of art, its creator and the person who paid for it. Yeats's great poem Meditations in Time of Civil WarThe poem was written during the Irish Civil War (1922 - 1923). addresses the paradox of something beautiful being commissioned by a powerful, violent man.

Many artists have led immoral lives and held shameful views, leading critics to argue that their work should be censored. Others, though, believe that the finished product is all that matters.

The Victorian critic John Ruskin argued that "perfecting the morality, or ethical state, of men" was one of the key functions of art. But his contemporary Walter Pater believed that the creation of beauty was more important.

For Herbert ReadA 20th-Century British poet and art historian., art and morality were two quite separate things: "Morality has only one aim - the ideal good; art has quite another aim - the subjective truth."

Some say, yes: nothing can be considered beautiful if its creation involves the suffering of others. Great art also has a moral purpose, which is completely undermined by the immorality of slavery. Not even the poorest artist should have accepted money offered by a slave trader.

Others say, no. Many artists have deep flaws, like most human beings, but in creating masterpieces they connect to something greater than themselves. Their work should be appreciated for its own merit: the circumstances in which it was created has nothing to do with its aesthetic value.