Prosecutor Mats Ljungqvist said "several people" were involved in the collection of the information and a "high level of expertise" was required.

Ljungqvist told AFP the suspect, Hans Georg Svanstrom, had "carried out military missions for the Swedish armed forces".

The indictment was revealed as Sweden moves closer to completing an application to join the NATO alliance following Russia's invasion of Ukraine.

The indictment said that between December 2015 and November 2017, Svanstrom drew up detailed maps of sensitive military sites across the country.

"It involves a large number of installations important for the capacity of Sweden to defend itself in case of war," the prosecutor said in a statement.

"The release of this information could threaten national security."

The classified information was found at Svanstrom's home at Vaxholm, near Stockholm.

About 40 pieces of evidence, including USB keys, cameras and files were seized and will be produced at the trial, the indictment said.