A Tudor mystery unravels: the fate of Lady Mary Grey, Queen Elizabeth's prisoner and a potential heir to the throne, has never been resolved. Now Leanda de Lisle tells all.(Biography)

At the Prime Minister's country residence at Chequers, scribbles on the walls of the 12-foot prison room bear testimony to the dreary misery of the woman Elizabeth I had kept there. An heir to the throne, a potential English queen, now buried in obscurity.

If Lady Mary Grey is recalled today, it is as a historical footnote. She was the dwarf who married a giant, the curious youngest sister of the famous Lady Jane Grey. But Mary was a more significant figure than her stature in the literature suggests. And my discovery of lost manuscripts has helped me lay to rest a Tudor mystery that may interest the next prime minister, whoever that is, as he gazes at Mary's portrait later this year. For centuries, no one has known what Queen Elizabeth did with poor Mary Grey's body, but I have discovered where she was laid to rest.

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When Elizabeth became Queen in 1558, Mary Grey followed her sister Katherine, the second of the three Grey girls, in line to the throne. This is not, of course, how history remembers it. Mary, Queen of Scots is the cousin we recall as Elizabeth's …