The head of the Danish secret service Lars Findsen must remain in custody.
This was decided by a court in Copenhagen.
According to Danish media reports and the Reuters news agency, Findsen is accused of passing on top secret information.
Findsen had already been arrested in early December for reasons that are still unknown.
His detention has now been extended by a further four weeks, reports the Danish public broadcaster DR. The court hearings lasted eleven hours.
Intelligence scandal of national scope
The case has sparked a scandal in Denmark, with several intelligence officials being charged with leaking state secrets.
It also revealed how the US intelligence agency NSA used Danish data to spy on senior officials in neighboring countries.
The case came to a head last month after it was revealed that both Findsen and former Defense Minister Claus Hjort Frederiksen had been arrested and charged with leaking state secrets.
Frederiksen said in a statement on January 14 that he "would never dream of doing anything that could harm Denmark or Denmark's interests".
col/Reuters