Denmark acted incorrectly by refusing to allow an independent assessment of whether the man in question can still be considered dangerous to others, the ECHR found.
The case represents a rare human rights judgement against the Scandinavian country.
The individual in question was sentenced in 1996 to safe custody (forvaring in Danish), a type of sentence which keeps him imprisoned with no time limit for as long as he is deemed dangerous.
He was given the sentence for attempting to sexually assault a ten-year-old child.
In the intervening years, the Herstedvester Fængsel prison, where he is held, has on repeated assessments found him too dangerous to be released on parole.
But not allowing an independent assessment is in breach of Article 5 of the European Convention on Human Rights, the ECHR found.
The Strasbourg court agreed in 2018 to review the case against Denmark, an unusual instance in itself, Ritzau writes.
Even rarer is an outcome in favour of the claimant.
“It is rare for Denmark to be given a mark on its human rights record,” said lawyer Tobias Stadarfeld of Aarhus-based firm Bonnez and Ziebe, who presented the case against Denmark at the ECHR.
“On a practical level, this means that the court agreed with us that a (human rights) infringement occurred, and that the process did not live up to the conditions of the convention,” Stadarfeld added.
The ruling means Denmark is now obliged to correct the issue by allowing an independent assessment of the prisoner.
The case can also be appealed with the ECHR’s highest court, the Grand Chamber.