Finland would provoke a "serious crisis" with Russia if it joined NATO, a report commissioned by the government warned on April 29.
The expert report also said that Finland and Sweden should move together if they want to join the transatlantic military alliance.
"Finland would be more exposed and vulnerable than it currently is if Sweden alone were to join NATO," it said, while "Finland joining NATO with Sweden staying out would create a strategically awkward situation."
"Membership would probably also lead to a serious crisis with Russia, for an undefined period of time."
The report was commissioned by Finnish Prime Minister Juha Sipila because of Russia's increased military activity in the Baltic region and its aggressions in Ukraine.
Finland shares a 1,340-kilometer border with Russia. It was attacked by its powerful neighbor during World War II but has maintain peaceful relations with Moscow since then.
NATO is open to Finnish membership, but Helsinki has been reluctant to join even as it cooperates with the alliance.
Sipila said his government has to be ready to seek NATO membership if necessary, adding: "With Sweden, we have promised not to surprise each other in these matters."
Based on reporting by AFP and dpa