After a marathon debate lasting a day-and-a-half, 188 of 200 members of parliament voted in favour of NATO membership, a dramatic turnaround from Finland's military non-alignment policy dating back more than 75 years.

"Tonight, we will sign the application letter for NATO", Foreign Minister Pekka Haavisto told public broadcaster YLE.

"Then possibly tomorrow, together with Sweden, the Finnish ambassador to NATO will submit the letter to NATO", he said.

Finland, which shares a 1,300-kilometre (800-mile) border with Russia, has been rattled by Russia's invasion of another neighbour, Ukraine.

"Our security environment has fundamentally changed," Finnish Prime Minister Sanna Marin told parliament on Monday at the start of the debate.

"The only country that threatens European security, and is now openly waging a war of aggression, is Russia", she said.

Finland spent more than a century as part of the Russian empire until it gained independence in 1917. It was then invaded by the Soviet Union in 1939.

Finns put up a fierce fight during the bloody Winter War, but were ultimately forced to cede a huge stretch of their eastern Karelia province in a peace treaty with Moscow.

According to public opinion polls, more than three-quarters of Finns want to join the alliance, almost three times as many as before the war in Ukraine began on February 24.

Sweden announced its official intention to apply for NATO membership on Monday and Foreign Minister Ann Linde signed the application letter on Tuesday.