French police have recovered a million-pound heirloom ring stolen from the fiancée of Prince Jean-Christophe Napoléon, a descendant of Napoléon Bonaparte and a pretender to the throne of France.

The ring, set with a dazzling 40-carat diamond taken from the crown of France’s last empress, Eugénie, was found partly thanks to the investigative efforts of the prince and his fiancée, Countess Olympia von Arco-Zinneburg.

It was stolen from the countess’s bag, which had been left in the couple’s unlocked car outside a Paris hotel. They parked their Mercedes GL last Monday afternoon in a street beside the five-star Hotel d’Aubusson in Paris’s plush 6th arrondissement, where they were meeting their parents.

The car was unlocked but they remained nearby and kept an eye on it. When they came to pick up the bag minutes later, however, it had vanished.

None of the hotel’s CCTV cameras showed the car, but after reporting the theft to police, the couple checked the countess’s bank accounts and found that one of her credit cards had been used minutes earlier at a sushi restaurant.

They phoned the restaurant, only to be told that the man who had paid with the card had just left. They then saw that he had used it to book himself into a nearby hotel, where police obtained an image of the suspect from CCTV.