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.