Princess Mary’s daughter has shocked royal watchers after being spotted at a music festival sporting an X-rated t-shirt.

Princess Isabella, 18, who usually adopts a more demure appearance, was attending one of Denmark’s most well known music festivals, Smukfest in Skanderborg, with her brother Crown Prince Christian and a group of friends when she was caught on camera.

Princess Isabella is the second child and eldest daughter of King Frederick and his Australian-born wife Princess Mary.

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Her choice of clothing has come under fire from some quarters, with claims it crosses the line of decency.

The black t-shirt was emblazoned with the words “f***ed him yesterday” with an arrow pointing to the side.

The top is thought to be merchandise for Danish hip hop group Suspekt, who the Princess had been watching at the festival. The trio are known for their explicit and sexual language.

Jarl Cordua, a Danish political commentator, was among a number of people to register their disgust with royal’s choice of attire.

“’F****d him yesterday’. The rich are having a party because the second-oldest royal child let loose at a festival yesterday wearing a shirt with the aforementioned text,” he wrote on social media in response to royalist and TV presenter Jim Lyngvild who said her choice of attire was “brilliant”.

“Not surprisingly, there are people who love that kind of thing. I’m also in favour of the royals letting loose a little. They always have. They should definitely be allowed to. BUT. There are limits.”

Cordua continued: “We’ll probably live with that shirt, but I wouldn’t recommend making it a habit. Being royal is an obligation. And it’s probably time for someone to tell her that.”

Lyngvild, who admits he was initially shocked by the t-shirt, said he soon changed his mind and called Princess Isabella’s boundary-pushing fashion moment “the best PR move for the Royal House in many years”.

“I expect a lot from her… I LOVE that our royal youngsters also have personality and courage – and don’t follow the neat pleated flow,” he wrote on Facebook.

“The time of the ladies in the hat is over, Princess Isabella is in.”