A LONG lost portrait of a young and healthy looking poet Samuel Taylor Coleridge ''before the opium kicked in'' has sold at auction for £51,000.

The location of the painting was unknown for decades until a seller put it up for sale, the BBC reports.

It was only known to exist because an engraved version was held at the National Portrait Gallery.

The portrait, measuring 28 cm by 24.5 cm and set in a gilt frame, depicts the 18th-century Romantic author Samuel Taylor Coleridge at a young age.

An inscription on the portraits frame , dated 1873, suggests it was painted at Nether Stowey, Somerset, where Coleridge rented a cottage between 1797 and 1800.

He is said to have lived there when he penned some of his best-known poems, including the Rime of the Ancient Mariner and Kubla Kahn.

Attributed to artist William Shuter, it had been estimated to fetch between £500 and £1,000 at Wotton Auction Rooms in Gloucestershire.

However, The picture was bought by an anonymous bidder from the UK, who will pay £62,000 for it including the buyer's premium.

Auction house cataloguer Julia Fry said she identified the painting because it was very similar to an existing portrait of Wordsworth.

She said the painting had been sold by a local family who had owned it for many years, but had not realised its significance.

Julia told the BBC: "It's a lovely portrait. It was nice to see Coleridge in a young state, before the opium kicked in."