Danish royal watchers are convinced the heir to their throne, Crown Prince Frederik, is about to announce his engagement to former Sydney real estate agent Mary Donaldson.

Ms Donaldson was wearing a large ring on her left ring finger when she was spotted in public with the crown prince in Denmark last weekend. The ring is the latest development in a romance that began two years ago at the Sydney Olympics and has since captivated Denmark.

Whether the ring signifies an engagement has Danish royal watchers in a frenzy of debate. The 30-year-old former Belle Property agent and her 34-year-old royal lover are becoming increasingly public with their affections.

While there is a question mark over the engagement, there is no doubt Ms Donaldson is being groomed to assume the role of future queen of Denmark. Gone are the casual outfits she used to wear before she left Sydney. In their place are smart suits like the white and beige ensemble she wore to the baptism, and stunning couture gowns like the one she wore to the wedding.