Queen Maxima said her eldest daughter, Crown Princess Amalia, "can't leave home" and that this has "enormous consequences for her life".

A visibly emotional Maxima said at the end of a state visit to Sweden on Thursday that the 18-year-old princess "doesn't live in Amsterdam and can't really go out".

She said that despite the security concerns, the princess is continuing her studies.

The Queen's words offered rare insight into the sense of unease felt by royal house.

It is also indicative of the fear shared by the Dutch and others across Europe that criminal gangs involved in the lucrative drug trade are proliferating.

The Dutch justice minister last week hosted a meeting at which six nations pledged to step up cooperation in the fight against organised crime.

That meeting came after security around Belgium's justice minister, Vincent Van Quickenborne, was beefed up following the arrest of four Dutch men in the Netherlands on suspicion of a plot to kidnap him.

Belgium and the Netherlands are home to the ports of Antwerp and Rotterdam, two key hubs for networks trafficking huge amounts of cocaine into Europe.

Another example underlining the gravity of the gang threat was a move by Dutch prosecutors this week to add terrorism to charges that a suspect faces over the killing of popular crime reporter Peter R de Vries last year.

Prosecutors said the suspect posted online a grisly video he recorded of De Vries after he was shot in an Amsterdam street to amplify the impact.

Amalia, 18, is studying Politics, Psychology, Law and Economics at Amsterdam University and was supposed to be living in a house with other students in the heart of the city's historic canal network.

Instead, she is still at home in nearby The Hague, her mother said. "She doesn't have a student life," Maxima said.

Prime Minister Mark Rutte, who reportedly also has had his personal security increased amid threats, called it "terrible news" for Amalia.

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