It narrowly escaped being melted down after the French Revolution, but a 16th century gold case containing the heart of the only woman to have twice been crowned queen of France has now been stolen.

The theft of the reliquary containing the heart of Anne of Brittany, who was briefly betrothed to the Prince of Wales, has provoked outrage over the loss of an object of enormous historical value.

Robbers broke in through a window of the Thomas-Dobrée museum in the western French city of Nantes during the weekend. They got away with the 6-inch oval case despite setting off an alarm.

Philippe Grosvalet, the president of the Loire-Atlantique department which owns the museum, said: “The thieves attacked our common heritage and stole an item of inestimable value. Much more than a symbol, the case containing the heart of Anne of Brittany belongs to our history.”

Mr Grosvalet said the theft was particularly disturbing as the reliquary had been saved from being melted down during the turmoil following the French Revolution in 1789.

The reliquary, topped by a gold crown with nine ‘fleurs-de-lis’, the lily-shaped royal motif, is considered a masterpiece. It was displayed at the museum for more than 130 years.

A picture shows the solid gold casket which contained the heart of the French queen Anne of Brittany, exposed by the castle of Blois, central France

Catherine Touchefeu, a departmental councillor, urged the robbers to return it. “If the thieves were motivated by the fact that it is shiny and made of gold, they should understand that its historical and symbolic value far outweighs its 100 grams of gold.”

After Anne’s death in 1514, she was buried, as custom dictated, alongside other French royals in the Basilica of Saint Denis outside Paris. But to show that her heart belonged to Brittany, it was placed in her parents’ tomb at the chapel of the Carmelite friars in Nantes, in accordance with her wishes. As queen she defended the autonomy of Brittany, then a duchy linked by treaty to France and often referred to as “Little Britain”.

Reputed to be the richest woman in Europe, her hand was eagerly sought by many kings. In 1483, her father arranged for her to marry the Prince of Wales, Edward, but the young prince disappeared, presumed to have been killed by his uncle Richard III.

She married Charles VIII of France in 1491, ascending the throne as queen consort at the age of 12. As he died without an heir in 1498, she married Louis XII a year later and became the only woman to be crowned queen of France twice.

Louis, grief-stricken when she died in 1514 at the age of 36, is said to have wept for eight days and ordered her tomb to made large enough for two.