According to top-selling Danish magazine Her og Nu, Queen Margrethe has a secret plan to abdicate on her 70th birthday on April 16.
If true, it would mean Frederik and Mary could be crowned king and queen within months.
Reports say the pair have already begun taking over some of Queen Margrethe's duties.
It's also thought the monarch is sure the Tasmania-born princess, 38 next month, and her husband are both ready for the roles.
How much weight the stories carry remains uncertain, however. This is not the first time speculation about the Queen's retirement has arisen ahead of her upcoming landmark birthday .
"There was a rumour a few months ago, but it had more holes in it than Swiss cheese," says Hellomagazine.com's source in Denmark.
"It would be a big surprise if she abdicates. Nobody is expecting it."
And things seemed very much business as usual at the banquet in Copenhagen's Christiansborg Palace on Wednesday.
Both Mary and Frederik looked relaxed as they arrived at the gala dinner, hosted by Queen Margrethe in honour of the country's politicians.
Mary wore an opulent blue velvet dress while her French sister-in-law Marie appeared alongside husband Prince Joachim in a light blue gown with gold details around the waist.
The evening's only out-of-the-ordinary occurrence came from altogether different source.
Just after the Queen and her husband Prince Henrik had passed, one of the royal guards in the line-up fainted dramatically on to the floor.
Knocked out by the fall, the unfortunate soldier had to be helped to his feet by security personnel.