Finland officially announced its intention to join NATO on Sunday as Sweden's ruling party said it backed membership, paving the way for a joint application.

The move is a dramatic turnaround from the two countries' military non-alignment policies, dating back more than 75 years for Finland and two centuries for Sweden.

In Helsinki, parliament began a marathon session that could last several days, following a membership proposal presented on Sunday by President Sauli Niinisto and Prime Minister Sanna Marin.

"Our security environment has fundamentally changed," Marin told parliament.

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

Russia has repeatedly warned Finland and Sweden of consequences if they apply to join NATO.

"This is another grave mistake with far-reaching consequences," Deputy Foreign Minister Sergei Ryabkov was quoted as saying by Russian news agencies on Monday.

An overwhelming majority of Finland's 200 MPs -- at least 85 percent -- back the decision to join NATO after Marin's Social Democratic Party on Saturday said it was in favour of joining.

Speaker of parliament Matti Vanhanen said "over 150 requests to speak were made" and the vote was not expected to take place on Monday.

Public opinion is also strongly in favour of membership.

According to recent polls, the number of Finns who want to join the alliance has risen to more than three-quarters, almost triple the level seen before the war in Ukraine began on February 24.

Finland, which shares a 1,300-kilometre (800-mile) border with Russia, was 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.

Meanwhile in Sweden, Prime Minister Magdalena Andersson said at the weekend she would consult parliament on Monday before announcing her government's official intention to apply.

That announcement was expected later Monday, according to media reports.

Her Social Democratic Party came out in favour of joining the alliance on Sunday, securing a firm majority in parliament for membership.

The turnaround by her party is dramatic, having opposed NATO membership since the birth of the alliance, with Andersson herself expressing opposition as recently as March.

Swedish public support for NATO membership has also risen dramatically, to around 50 percent -- with about 20 percent against.