A rare Tudor portrait of Margaret Beaufort, the formidable grandmother of Henry VIII, has been unveiled by the historian David Starkey at Hever Castle in Kent.

Beaufort married three times – a fourth marriage when she was only a child was annulled – and outlived all her husbands. Her ambition helped propel both her son and her grandson to become kings of England.

Starkey has called Beaufort the most powerful woman in England of her day: after her son was crowned she signed her letters Margaret R. She was descended from Edward III through John of Gaunt, and was also a scholar and author who founded two Cambridge colleges.

She lived just long enough to see Henry VIII crowned; Starkey has suggested that overeating at the coronation banquet may have contributed to her death.

Although she appears meek and nun-like in the painting, she was one of the wealthiest women in the country. The apparent simplicity is deceptive: she is portrayed holding an illuminated prayer book, a luxury object, against a sumptuous silk cloth of state.

The picture, recently acquired by the castle to add to an important collection of Tudor portraits, was painted long after her death in the reign of Elizabeth I, possibly copying an original which is now lost.

It will be placed on display as Hever, the childhood home of Anne Boleyn, Margaret's granddaughter-in-law, reopens for the season.