According to the prosecutor, the article contained harmful information the publication of which was against the law.

Helsingin Sanomat Editor-in-Chief Kaius Niemi, who had charges against him in connection with the case dropped for lack of evidence, said the journalists had not broken the law.

"We can show for each published sentence that the information could be found on the internet or in books prior to the publication of our article. Public information cannot be classified," he told Reuters.

The three deny any wrongdoing.

Finland has for years been among the top countries in a global press freedom ranking published annually by Reporters Without Borders.

But it slipped to a fifth position this year, partly due to the court case, the journalist association's Finnish branch said.

The journalists were not present at the preparatory session at Helsinki district court on Thursday.

Hanne Aho, chairwoman of the Finnish Journalists' Union, said the case was first of a kind.

"It is completely exceptional that Finnish journalists are being accused of high treason," she told Reuters.

Under Finnish law, it is customary to name defendants in a criminal case only if they are convicted.