Finland is prepared to make a possibly historic choice on whether to apply to join NATO as a "deterrence to Russian acts" before midsummer.

The 5.5 million-strong Nordic nation has long been non-aligned militarily, in part to avoid offending its eastern neighbor, with whom it shares a 1,300-kilometer (830-mile) border.

However, Russia's military operation in Ukraine on February 24 saw public support for joining NATO double from 30 to 60%, according to a series of polls.

"Never underestimate the capacity of Finns to take rapid decisions when the world changes," former Finnish Prime Minister Alexander Stubb told AFP.

Read more: Could Finland's NATO application be imminent?

Stubb, a long-time NATO supporter, now believes Finland's bid for membership is "a foregone conclusion" as Finns re-evaluate their relationship with their neighbor.

A government-commissioned national security review will be given to Finland's Parliament, the Eduskunta, next week to assist Finnish MPs in making their own decisions before it is submitted to a vote.

"We will have very careful discussions but not taking any more time than we have to," Prime Minister Sanna Marin told a news conference on Friday.

"I think we will end the discussion before midsummer," she added.

"My guess is that the application will be filed sometime during the month of May" in time for the June NATO summit in Madrid, Stubb said.

Anti-NATO Finns

In a recent poll conducted by national broadcaster Yle, just six of Finland's 200 MPs publicly expressed anti-NATO sentiments, including Markus Mustajarvi of the Left Alliance party.

The non-alignment of Finland and Sweden "has delivered stability to entire Northern Europe," the Lapland MP told AFP.

Mustajarvi wonders if NATO's Article 5 pledge to mutual defense will give actual security in the event of an attack.

Instead, he describes Finland's defense capabilities as "so robust that they would force Russia to consider the cost of attacking."

Despite receiving "all sorts of feedback" from the public and his fellow MPs over his stance, Mustajarvi insists he has "thought this through to the end and so far I don't see a reason to change my position".

Grey zone 

Since Russia's military operation, Finland's leadership has undertaken an intensive series of talks to canvass opinions from other NATO states about a possible membership bid.

Finland, along with neighboring Sweden, has received public assurances from Secretary-General Jens Stoltenberg that the alliance's door remains open, as well as support from several members, including the United States, United Kingdom, Germany, France, and Turkey.

Attempting to join NATO, on the other hand, would almost certainly be viewed as a provocation by the Kremlin, which sees the expansion of the US-led alliance on its borders as a major security concern.

The Kremlin has pledged to "rebalance the situation" in the event of Finland joining NATO.