The portrait of Mary Seacole lay hidden for many years

Seacole, who rivalled Florence Nightingale for her feats in the Crimean War, was the daughter of a Scottish soldier and Jamaican mother.

The oil painting of the Jamaican-born nurse lay unseen for years.

The 9.5ins by 7ins painting will now go on display at the National Portrait Gallery in London.

The artwork was painted by London artist Albert Challen and dates from around 1869 showing an older Seacole wearing a red neckerchief and the three medals which she was awarded for service.

I am extremely happy that she can at last take her rightful place in British history as an important female personality of the Crimean War Helen Rappaport Owner

There are no other known painted portraits of Seacole, who died in 1881 in her London home.

Seacole was voted Greatest Black Briton in an online poll last year, while 2005 marks the bicentenary of her birth.

The first step in its discovery began when a dealer, curious about the inscription AC Challen 1869 on the work, unsealed the frame.

Apparently unaware of what he had found, the dealer sold the portrait at a local auction in Warwickshire.

After being approached about the nature of the medals on the figure by another dealer, historian Helen Rappaport immediately recognised the identity of the sitter, bought the portrait and took it to the National Portrait Gallery for examination.

Ms Rappaport, the portrait's owner, said: "As an admirer of Mary Seacole's courage and humanitarianism, I am extremely happy that she can at last take her rightful place in British history as an important female personality of the Crimean War."

Genuine article

Experts at the gallery believe the details of Seacole's dress and the portrait's pigments show the painting is genuine.

Sandy Nairne, director of the National Portrait Gallery, said: "This is a wonderful discovery.

"A painted portrait allows us to appreciate the important 19th-century figure of Mary Seacole in new ways."

Seacole remained in the Crimea until 1856 with a reputation that rivalled that of Florence Nightingale.