Il Nuovo Postiglione - Frederik, from party prince to respected king of Denmark

Passionate about the environment, the 55-year-old has discreetly imposed himself in the shadow of his hugely popular mother, Queen Margrethe II, who announced her plans to abdicate in a televised New Year's Eve address after 52 years on the throne.

Enjoying the support of more than 80 percent of Danes according to a recent poll, the prince will become King Frederik X upon his mother's abdication on Sunday.

"When the time comes, I will guide the ship," he said in a speech celebrating his chain-smoking mother's half-century on the throne in 2022.

"I will follow you, as you followed your father" in leading the thousand-year-old institution, Prince Frederik added.

But this measured assurance is a far cry from his younger self.

"He was not strictly speaking a rebel, but as a child and young man, he was very uncomfortable with the media attention and the knowledge that he was going to be king," said Gitte Redder, an expert on the Danish royal family.

He only "gained confidence in his mid-20s," she told AFP.

- Lonely and tormented -

A lonely and tormented teenager, Frederik resented his parents for neglecting him as they fulfilled their royal obligations.

He sought solace in fast cars and fast living, and was considered a spoiled party prince in the early 1990s.

But that view began to change after he graduated from Aarhus University in 1995, the first Danish royal to complete a university education.

His time at college included a stint at Harvard in the United States, where he was enrolled under the pseudonym Frederik Henriksen.

The fake surname was a nod to his father, French diplomat Henri de Monpezat who became Prince Consort Henrik when he married Margrethe.

But Frederik -- who in addition to Danish speaks English, French and German -- really began to mature into his role during his time training in the three branches of Denmark's military.

The prince served in the navy's Frogmen Corps -- where he was nicknamed "Pingo" (Penguin) -- one of only four of the 300 recruits to pass all of the tests in 1995.

In 2000, he took part in a four-month, 3,500-kilometre (2,175-mile) ski expedition across Greenland.

- Complementing the queen -

His daredevil side has landed him in hospital after sledging and scooter accidents, but his popularity has soared, boosted by the Royal Run, annual fun runs across Denmark that he began in 2018.

"He is a sportsman, he attends concerts and football matches, which makes him even more accessible than his mother," royal expert Redder said.

"I don't want to lock myself in a fortress. I want to be myself, a human being," he once said, insisting he would stick to that even after taking the throne.

He met his wife Mary Donaldson, an Australian lawyer, in a Sydney bar during the 2000 Olympic Games.

They have tried to give their four children as normal an upbringing as possible, sending them mainly to state schools.

Their eldest, Prince Christian, who recently turned 18, was the first Danish royal to go to daycare.

The couple have gradually taken on many royal duties in recent years as the queen entered her eighties, "but very slowly and depending on the queen's health", said historian Sebastian Olden-Jorgensen.

The couple are "modern, woke, lovers of pop music, modern art and sports," he added.

They "do not represent a potential revolution compared to the queen", but a careful transition adapting to the times, he said.

Among other things, Frederik has championed Denmark's drive to find solutions to the climate crisis.

He has said that he sees himself complementing his mother, a polymath who is an accomplished writer and artist.

"You paint, I exercise. You dig for buried objects from the past, I buried my head in order not to be recognised during my time in the armed forces. You are a master of words. I am sometimes at a loss for them," he joked during the queen's jubilee celebrations.

A.Imbrogno--INP