A foreign ministry statement said the move would seriously damage bilateral relations, as well as security and stability in northern Europe.
Earlier, Finland’s president and PM called for the country to apply for NATO membership “without delay”.
It comes amid a surge in public support for NATO membership following Russia’s invasion of Ukraine.
Finland shares a 1,300-km (810-mile) border with Russia. Until now, it has stayed out of NATO to avoid antagonising its eastern neighbour. Finland will formally announce its decision on Sunday after it has been considered by parliament and other senior political figures. Read more here.