He's behind you! Elizabeth Vernon, Countess of Southampton by an Unknown artist, circa 1620. © National Portrait Gallery, London

Students researching a new display of Tudor portraits in the collection of the National Portrait Gallery have uncovered a ghost figure, which may be Shakespeare's only known patron, Henry Wriothesley, 3rd Earl of Southampton.

The discovery was made when the portrait was x-rayed for a display of portraits at the National Trust property Montacute House, Somerset, curated by students from Bristol University.

The portrait had been subsequently painted over with a portrait of Elizabeth Vernon, Southampton's wife, by the unknown artist, possibly because a commission for a double portrait of husband and wife was abandoned in favour of the single portrait seen today.

A favourite of Elizabeth I, Henry Wriothesley, (1573 -1624), was the only known patron of Shakespeare, who dedicated his poem Venus and Adonis to him (1593). Some hold the theory that Shakespeare's sonnets were addressed to him.

Southampton's tempestuous relationship with the Queen culminated in his involvement in Essex's rebellion in 1601. Condemned to death when the rebellion failed, his punishment was commuted to life imprisonment and he was released by James I.

The x-ray shows the portrait behind the Countess of Southampton. © NPG

The portrait shows Elizabeth Vernon, a maid of honour to Elizabeth I, who was involved in an intrigue with Henry Wriothesley in 1595. She married him in secret three years later.

She is shown in her forties after several decades of marriage. She wears a black dress slashed to reveal scarlet fabric, a white lace coif adorned with pearls and a scarlet flower to match.

Her jewels include ruby earrings and a ring on the little finger of her right hand. The 'S' on her chain presumably stands for 'Southampton' and suggests that the miniature locket which she wears on her chest may contain a portrait of her husband.

Another very similar version of this portrait, attributed to van Somer, exists at Sherborne Castle, Dorset, very close to Montacute House, where this portrait can now be seen.

On the Nature of Women: Tudor and Jacobean Portraiture 1535-1620 runs until October 1 2009.