With a history of bad health, Margrethe, who turns 79 on April 16, dropped hints that she now considers her son ready to take over.
“He is fantastic – much better than me,” she said in Buenos Aires during their three day tour.
The Queen, who has ruled for 47 years, has always made it clear she will die in office, and there is no tradition of abdication in Denmark, unlike other European countries.
But times are changing and polls suggest the public feels she deserves to step down and enjoy a rest in her later years.
While the Queen and Frederik were in South America, Mary continued to boost her credentials with a trip to Texas, promoting Danish culture, green energy and trade.
She then flew to Addis Ababa as a UN ambassador for a two-day visit promoting women’s rights in Ethiopia.
Mary talked about gender equality and met with local business leaders and female movers and shakers in the rapidly developing African country.
Back in Denmark, our Aussie royal icon, now 47, took on yet another role as she was named captain of the Danish Home Guard.
In full uniform, Mary marched confidently at the base in Nymindegab, Southwest Jutland, to receive the honour as the Queen – freshly back from her trip – looked on approvingly.
Royal reporter Jacob Heinel Jensen of Se Og Hoer magazine said: “The trip to Argentina was clearly designed as a kind of exam for Frederik to see if he is ready to be king, and he passed with flying colours.
“You could see how proud the Queen was of him and how much respect he has for her as he looked to her for advice on what to do and how to talk to people.
“The Queen has not been in great health for some years and the last state visit she carried out was in Ghana in 2017, so I can’t see her doing any more.
“This was probably her last trip, at least long-haul, and she is clearly ready for Frederik and Mary to take the reins.
“Having said that, incredibly she seemed to be in fine form and good health on the trip.
“She had recently had a holiday in Norway with the royals there, and she was coping well in Argentina.
“One day she had a packed schedule and a long train trip. A reporter asked if she would take a nap and she replied: ‘Why would I? I won’t get rid of my jet lag!’
"She looked better than in recent years. So maybe she is finally coming round to the idea that she could abdicate and enjoy a few years in retirement while her health holds up.
“Certainly the Danish people would like her to do that.
“I don’t have any doubt she feels Frederik is ready with Mary by his side.
“She thinks Mary is a very strong woman who brings out the best in her son.
“Mary doing these trips abroad and taking on more patronages, particular with the armed forces, shows she is already the de facto Queen in many people’s eyes.
“The Queen has spent the last decade or so training up Mary because she is so important for the future, and she is delighted with her progress.”
Two years ago Margrethe heaped praise on Mary in a book interview saying: “She’s brilliant. She can do things that I was not able to do at her age.
“I have to say that I’m so happy with the Crown Prince’s choice. She is very good for her husband and you can see he thrives on it!”
Constitutional expert Sebastian Olden-Jorgensen of the University of Copenhagen said: “I can’t see the Queen abdicating in 2019.
“She will want to stay on until at least 2020 when there is a big national celebration to mark 100 years since part of Denmark was returned from Prussia.
“But there is no doubt that Frederik and Mary are stepping up their duties and increasing the importance of their roles as the Queen ages.
“Maybe when she’s 80 would be a good time to hand over.”
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