Finland's president and prime minister have said they are in favour of rapidly applying for Nato membership "without delay," paving the way for the alliance to expand amid Russia's war in Ukraine.
The dramatic move by Finland was announced by President Sauli Niinisto and Prime Minister Sanna Marin.
It means that Finland is all but certain to join Nato, though a few steps remain before the application process can begin.
Neighbouring Sweden is expected to decide on joining Nato in the coming days.
The Kremlin reacted to the development a few hours later, saying that Finland's move to join Nato will not help stability and security in Europe.
Finland shares an 830-mile land border with Russia.
Kremlin spokesman Dmitry Peskov said that Russia's response to the move would depend on what specific steps Nato will take to bring its infrastructure close to Russian borders.
He noted that Russian President Vladimir Putin already had ordered to work out steps to strengthen the country's defences in the west in response to Nato's expansion closer to Russian borders.
Previously, the Kremlin had warned of "military and political repercussions" if Sweden and Finland decide to join Nato.
Should they apply, there will be an interim period lasting from when an application has been handed in until all 30 Nato members' parliaments have ratified it.
"Nato membership would strengthen Finland's security. As a member of Nato, Finland would strengthen the entire defence alliance," Mr Niinisto and Ms Marin said in a joint statement.
"Finland must apply for Nato membership without delay. We hope that the national steps still needed to make this decision will be taken rapidly within the next few days."
The statement on Thursday came a day after Prime Minister Boris Johnson visited both Finland and Sweden to sign a military co-operation agreement.
The UK pledged on Wednesday to come to the aid of Sweden and Finland if the two Nordic nations came under attack.
During a joint news conference with Mr Johnson and Mr Niinisto in Helsinki, the Finnish head of state said Moscow could only blame itself should his nation of 5.5 million people become a Nato member.
"You (Russia) caused this. Look at the mirror," Mr Niinisto said pointedly on Wednesday.
On Thursday, Mr Niinisto tweeted that he spoke with Ukrainian President Volodymyr Zelensky about Finland's firm support for Ukraine and the country's intention to join Nato.
Mr Niinisto said that Mr Zelensky "expressed his full support for it".
In 2017, Sweden and Finland joined the British-led Joint Expeditionary Force, which is designed to be more flexible and respond more quickly than the larger Nato alliance.
It uses Nato standards and doctrine, so it can operate in conjunction with Nato, the United Nations or other multinational coalitions.
Fully operational since 2018, the force has held a number of exercises both independently and in cooperation with Nato.
Since Russia's invasion of Ukraine on February 24, Finland and Sweden have been pondering whether to abandon their historic, decades-old neutrality and join the 30-member Nato.
After Moscow launched its attack on Ukraine, public support in the two countries started to quickly shift toward membership in Nato, first in Finland and a bit later in Sweden.
The latest opinion poll conducted by Finnish public broadcaster YLE showed earlier this week that 76% of Finns are in favour of joining Nato, a big change from earlier years when only 20-30% of respondents favoured such military alignment.
Speaking to European Union politicians on Thursday as Mr Niinisto's and Ms Marin's announcement was made, Finnish Foreign Minister Pekka Haavisto said that "the war started by Russia jeopardises the security and stability of the whole of Europe".
Mr Haavisto said that Russia's unpredictable behaviour is a serious concern for Finland, notably Moscow's readiness to wage "high-risk operations" that could lead to many casualties, including among Russians themselves.
Along with Sweden, Finland joined the European Union in 1995 and has the longest border with Russia out of all the bloc's 27 members.
Swedish Foreign Minister Ann Linde tweeted that Finland's announcement gave an "important message" and Danish Prime Minister Mette Frederiksen said that there were "strong messages" from Finland's president and prime minister.
During the Cold War, Finland stayed away from Nato to avoid provoking the Soviet Union, instead opting to remain a neutral buffer between the East and the West while maintaining good relations with Moscow and also with the United States.
Nato Secretary-General Jens Stoltenberg has said the military alliance would welcome Finland and Sweden - both of which have strong, modern militaries - with open arms and expects the accession process to be speedy and smooth.