If you think the beauty myth is oppressive nowadays, picture what it was like to be a woman in Renaissance Europe. The Renaissance created a cult of female beauty so all-pervasive that it shapes western perceptions and fantasies right down to today. From Botticelli's Venus to Titian's Flora, the greatest artists dedicated their genius to imposing impossible standards of beauty on a world that, in reality, was scarred by pox, ravaged by poverty and untouched by hygiene.

No wonder the tiny minority of women who got to control their image instructed painters to preserve their youthful good looks, or fabricate a finer face than they ever had. Only the handful of women who ruled states had any such power. Titian's portrait of Isabella d' Este, marchioness of Mantua, which hangs in Vienna's Kunsthistorisches Museum, shows her as a twentysomething beauty, soft skin unblemished by wrinkles, in the bloom of youth. In reality, she was about 52 when Titian painted it.

Elizabeth I ruled a much bigger territory than Isabella, and got a law made in England prohibiting the circulation of unflattering portraits of her. Elizabeth's portraits are notoriously fictitious in always showing her as a pearly-skinned icon of Renaissance beauty even when she was old. This is what makes a newly revealed portrait of her from the workshop of Marcus Gheeraerts so remarkable.

The portrait shows an unmistakably ageing Elizabeth, her wrinkles unconcealed by makeup, with heavy, dark lines under her eyes. The reality of fleshly deterioration and melancholy age is revealed almost as brutally as in a notorious portrait of the present Elizabeth by Lucian Freud. The glittering crown on Elizabeth I's head in the 1590s painting, the extravagant lace collar and jewels seem almost ironic surroundings for a face that is not just time-marked but miserable. Uneasy lies the head that wears the crown, as one of her subjects wrote.

Marcus Gheeraerts, whose studio seems to have produced this portrait, usually gave Elizabeth a far more idealised aspect. In his glittering Ditchley Portrait, she stands in a silver dress on top of a map of southern England, a beautiful colossus seen against a stormy sky in which the sun is breaking through. The story behind this painting reveals the culture in which Elizabeth had to be beautiful. It was commissioned by the courtier Henry Lee after Elizabeth got angry with him for taking a mistress: the painting commemorates her forgiveness. She is the fairy queen, taking pity on Lee.

It is part of a game called courtly love: to be one of the unmarried Elizabeth's favourites you had to participate in a chivalrous ritual courtship of her. A miniature portrait in the V&A, by Nicholas Hilliard, shows a handsome young man sporting emblems and symbols of Elizabeth as he poses, lovestruck, in a garden: clearly to justify such love the Queen had to be seen as beautiful.

So it was not vanity that made Elizabeth insist on beautiful portrayals of her such as George Gower's that was painted to celebrate the defeat of the Spanish Armada. Daring sailors who participated in that battle, like Sir France Drake, flirted with their Queen and played the game of courtly love with immense dash. Of course, their love object was a beauty. This was fundamental to her ability to rule such men.

And yet, when you look at the beautiful portraits of Elizabeth more closely, they tend to smooth and whiten her face rather than radically reinventing it. Realism was burned into the British idea of art by Hans Holbein, who was court painter to Elizabeth's father Henry VIII. Portraits were expected to look like the person they portrayed. It was a fine line an artist had to walk, between flattery and fact. A portrait must please the sitter and – in the case of royalty – promote a public image without obviously being fantastical. In Elizabeth's portraits it is actually her clothes, jewels and hairstyles that create glamour. Holbein himself perfected this trick.

In the new, unvarnished portrait of Elizabeth I, wrinkles-and-all, the artist has stepped over a fine line. All the accoutrements of her glamour are there, but the painter has gone just that bit nearer to the reality behind the myth than was required to give a portrait plausibility. The result is a cruel unmasking of power. Could this have been a deliberately subversive image, hidden away in the house of some rebellious lord? Here is the fairy queen, her spell broken.