Brooch identifies portrait as Lady Jane Grey, the girl who reigned for nine days

For decades, the female sitter in this 16th-century miniature painting remained unidentified. But after investigating the woman's jewellery, the historian David Starkey is now convinced this is the only known portrait of Lady Jane Grey painted during her lifetime to have survived.

It may have been produced when Lady Jane, the queen who ruled for nine days, was living in royal lodgings at the Tower of London where, instead of being crowned, she was executed in February 1554.

The tiny picture will go on display tomorrow as part of an exhibition of newly discovered royal Tudor portraits organised by the London art dealer Philip Mould.

The remarkable discovery is likely to fuel speculation about a painting believed to be Lady Jane Grey, but painted after her death, which was acquired by the National Portrait Gallery for £100,000 last year.

Dr Starkey first noticed the miniature, which belongs to the Yale Centre for British Art, when he saw an illustration of it in a book. It was described as depicting an unknown woman.

"This one struck me instantly and I thought that it had to be Lady Jane. Almost all of the early miniatures such as this were of royal subjects. [The form] was practically reserved for royalty," he said.

"What I noticed was the evident youth of the sitter. It would be unusual for someone to sit for a miniature unless they had high status."

He used the brooch and the spray of jewellery "foliage" of the sitter as the starting point of his research, using inventories of the period.

"Following my work on the two big exhibitions - Henry VIII and Elizabeth I - I became profoundly aware of the importance of inventories," he said.

One drawn up for Henry VIII had previously helped the historian to establish that a portrait long believed to be Jane Grey was, in fact, Catherine Parr, the king's final wife; another of Catherine Howard, whose identity had been questioned, really was his fifth queen.

In this case, he turned to The Life and Letters of Lady Jane Grey by Nicholas Harris Nicolas, published in the early 19th century, which contained a copy of an inventory, the original of which is now lost.

It detailed the jewellery Lady Jane owned in July 1853 when she went to the Tower of London, where by custom, royals resided between accession and coronation, although it was to be her prison. The list included a gold brooch with an agate centre that matched the one in the miniature.

Further research showed that the foliage, which art historians have traditionally interpreted in terms of plant symbolism, was the badge of the family of John Dudley, Duke of Northumberland.

He was, in effect, the ruler during the last days of the boy king Edward VI whose death propelled Lady Jane briefly to the throne.

And it seems likely that it was to maintain his influence that Jane was forcibly married to his son, Guildford Dudley, in May 1553.

"Either it is a wedding picture or, more likely, it dates from the time that Lady Jane was in the Tower," Dr Starkey said. "It is the first confirmed image of her painted in her lifetime."

It is thought to be the work of the 16th century female miniature painter Lavinia Teerlinc.

Philip Mould is including the work in a two-week exhibition, 'Lost Faces - Identity and Discovery in Royal Tudor Portraiture', which opens tomorrow at his gallery in Dover Street, London. Unusually for a commercial gallery, the show includes loans from institutions and private collections and will be presented with a full academic catalogue.

Tragic queen

* Lady Jane Grey was born in 1537, a great-granddaughter of Henry VII of England.

* At the age of nine, she was sent to live with Catherine Parr, the sixth wife and eventual widow of Jane's great-uncle, Henry VIII. A clever, studious girl, she was educated beside the future Edward VI.

* In 1553, she was married against her will to Guildford Dudley, whose father, John, Duke of Northumberland, in effect ruled the country during the reign of Edward VI.

* Edward VI died in July 1553 aged 15 and, amid fears that the throne would go to his half-sister, the Roman Catholic Mary, Jane Grey, a staunch Protestant, was supported by many as a claimant to the throne. She reigned for only nine days before Mary became Queen.

* Initially, Mary spared Jane, and her husband Guildford, but after the Wyatt rebellion in January 1554, felt compelled to crush any Protestant dissent. Jane was executed on 12 February 1554.