Lars Findsen, 57, is among four current and former employees of the Danish intelligence services who were detained in December over the alleged leak.

Now Findsen is the only suspect to remain in custody while the Danish authorities continue to investigate.

Unconfirmed reports by state broadcaster DR suggest classified material was being passed on to Danish media outlets.

However, the case is being conducted behind closed doors - meaning the exact charges and details of the sensitive information have not been divulged publicly.

At a hearing in Copenhagen on Monday, Findsen described the situation as "completely insane".

He told reporters: "I want the charges brought forward and I plead not guilty."

The news was revealed by Danish media after the court lifted a reporting ban.

A judge is due to decide how long Findsen will remain in detention.

DR claimed in 2020 that the Danish Defence Intelligence Service shared raw data from information cables with the US National Security Agency - potentially giving the organisation access to Danish citizens' personal data and private communications.

The public prosecutor's office in Denmark has declined to comment on the case.

In a separate case, the spy chief and four other officials were suspended in August 2020 after facing accusations of serious wrongdoing by an independent board overseeing the intelligence unit.

But the suspensions were lifted after an investigating commission rebuffed the allegations.

Findsen previously led the Police Intelligence Service until 2007 before being appointed head of department at Denmark's Ministry of Defence.

He became head of the Danish Defense Intelligence Service in 2015.