"It's extraordinary and wonderful that this painting has emerged," said Sandy Nairn, Director of the National Portrait Gallery. "She's a really extraordinary figure and a great achiever."

Mary Seacole was born in Jamaica of a Scottish soldier and a mixed-race mother. Her mother ran a boarding house for invalid soldiers and it was from her that Mary learnt her nursing skills.

"It's also a great moment because it happens to be a time when other Mary Seacole events are happening," Mr Nairn added. The bicentenary of Mary's birth will be marked in 2005 by a series of events and exhibitions and a new biography will be published this January.

The portrait, sealed beneath a framed print, was bought at a car boot sale by a dealer, who noticed the inscription 'AC Challen, 1869' on the back. He removed the print to reveal the oil painting of Mary in a red neckerchief and wearing the three medals she was awarded for her service.

Seemingly unaware of what he had found, the artwork was sold on at a local auction in Warwickshire and found its way to writer and historian Helen Rappaport for identification.

"It was sent to me for confirmation," said Helen, who bought the portrait in 2004, six months after it had come to light. "I knew immediately that it was Mary Seacole." The identification was supported by likeness to a bust and a photograph of Mary and by details known of her dress.

"I'm thrilled and absolutely delighted that this portrait has been made available," Helen continued. "Every biographer dreams about finding a cache of undiscovered letters or other material."

"By a miracle this painting has survived – it's a wonderful day for all those people who have been campaigning to raise the profile of Mary Seacole and other black people. Her picture is going to hang in this gallery, alongside Queen Victoria and Florence Nightingale, where she jolly well belongs!"

Mary sailed to England in 1854 after the outbreak of war with Russia, and volunteered herself as a nurse. She was denied an interview with the War Office, who were recruiting nurses, probably on grounds of her skin colour.

Determined to help the troops, Mary travelled to the Crimea at her own expense and set up 'The British Hotel' outside Balaklava, where she tended to the sick and wounded. She became widely known as 'Mother Seacole', a familiar face on the battlefield and was later awarded the British Crimean medal, the Turkish Medjidie and the French Legion of Honour.

Comparisons are often made between Mary and the Lady with the Lamp, but Florence Nightingale did not associate with Mary, perhaps because of Victorian propriety – Mary sold alcohol to soldiers.

"Mary wasn’t a black Nightingale," explained Helen. "She operated in a completely different way, with a totally different agenda."

The artist to whom the picture is credited was only 22 years old in 1869 and is listed as an art student, living in Hammersmith, in the census of 1871. It is assumed that he or his family must have had some connection to Mary, but the provenance of the painting is difficult to trace.

Helen had to research Albert Charles Challon genealogically: "What is poignant is that this artist is completely unknown," she said, adding that "he died aged only 34, just months after Mary."

The painting, which was found in good condition, is on loan to the gallery indefinitely.

"I can only hope this will inspire others to research black history, especially black women’s history," said Helen, who would also like to hear from any members of the Challon family, last recorded in Stoke Newington in the 1920s.