Finland's Security Intelligence Police Supo will not release the so-called Tiitinen list to a researcher, the agency announced on Monday.
The list, which contains the names of 18 Finns who had contacts with the East German Stasi intelligence service between 1969 and 1990, has been sealed since it was passed to Supo in 1990 following the fall of the Berlin Wall.
At that time Supo was headed by Seppo Tiitinen, who later became the Secretary General of the Finnish parliament.
Researcher Kimmo Elo had asked for the list, was granted permission to study material related to Finnish-GDR relations during the Cold War, but will now be denied the list itself.
Supo said its release could endanger national security and its ability to co-operate with partners abroad.