The Nordic country had already revealed in October that it had been the target of "massive" digital spying without mentioning the origin of the espionage efforts.
At a press conference held on Wednesday in Helsinki, Finnish Security Intelligence Service (Supo) director Antti Pelttari accused "governmental agents" of being behind these attacks, which in some cases were "very sophisticated".
Pelttari refused to mention any specific countries, and did not disclose the content of the compromised filed.
The hackers "could have accessed information which might have influenced several negotiations" of the government, said foreign ministry official Peter Stenlund, adding it "obviously harms Finland's national interests".
However, the hackers did not access any files from European institutions or classified documents, which are stored in other computer systems which were not hacked, the foreign ministry said.
The hackers managed to install on the foreign ministry computers surveillance programmes which were sending files abroad.
Supo said that the investigation of the spying case was completed for the most part and that reinforced security measures had permitted to block other attacks.
"According to our information, several diplomatic networks in Western Europe have been targeted by this same type of spying for many years," Stenlund said.
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