A Dutch court on Thursday dropped the case against a father who isolated his children in a remote farmhouse for a decade, saying that he was mentally unfit for trial.

The 68-year old, identified only as Gerrit Jan van D., was arrested in October 2019 for allegedly keeping his children in a room on his farm in the northeastern village of Ruinerwold.

But tests had found that a stroke in 2016 had left the former cult member partially paralysed, unable to speak and cognitively impaired, prosecutors said last month, urging judges to drop the charges.

"The court is of the opinion that the defendant has such physical and cognitive limitations that he is unable to follow the criminal proceedings or to sufficiently understand the possible consequences of this process," the Assen District Court said.

"All things considered ...the public prosecutor's case should be declared inadmissible."

Five of the children were discovered on the remote farm after a sixth walked into a local bar in a confused state and raised the alarm.

The father faced kidnapping charges as well as claims of sexually abusing two of his three older children, who were not kept on the farm.

Gerrit Jan van D was arrested in October 2019 for allegedly keeping his children in a room on his farm for around 10 years
Gerrit Jan van D was arrested in October 2019 for allegedly keeping his children in a room on his farm for around 10 years ANP / Wilbert Bijzitter

But the judges said he was unable to discuss the case with his lawyers and put together a defence strategy.

"The suspect is insufficiently able to communicate... and during the court hearing he cannot have someone else put forward his reading of the facts and what he considers important for his defence," they said.

Prosecutors last month said they understood the decision to shelve the case would be "particularly hard" for the older children, who had helped the investigators.

But they added their request went to the heart of a fair trial.

Prosecutors said the father had isolated his children from birth and beaten them from a young age to drive out "bad spirits".

One was tied up by his hands and feet as punishment while another was forced to spend an entire summer in a doghouse.

Dutch media have said the family was "waiting for the end of time" as part of a doomsday cult.