Finland’s president and prime minister have vowed to join Nato, an announcement that will redraw Europe’s geopolitical map and push neighbouring Sweden to apply for membership in the aftermath of Russia’s invasion of Ukraine.

President Sauli Niinistö and Prime Minister Sanna Marin both said on Thursday that Finland “must apply” for Nato membership within days.

“Nato membership would strengthen Finland’s security. As a member of Nato, Finland would strengthen the entire defence alliance,” they said in a joint statement.

Russia’s bloody war against Ukraine, another non-Nato European nation, has altered the security considerations of Finland and Sweden and led to large majorities backing membership of the western defence alliance.

Marin’s Social Democrats will announce their decision on Saturday, with the five-party coalition government in Helsinki expected to follow on Sunday.

In Sweden, the ruling Social Democrats will make a decision on Sunday whether to join Nato. Formal applications by both countries are expected next week.

“We hope that the national steps still needed to make this decision will be taken rapidly within the next few days,” Niinistö and Marin said.

For years, public backing of Nato membership hovered between 20 and 30 per cent in Finland even after Russia’s incursions into Georgia in 2008 and Ukraine in 2014. But the most recent poll showed 76 per cent of Finns back joining Nato, and only 12 per cent opposed.

Asked on Wednesday what his message to Russia would be, Niinistö said: “My response would be: you caused this. Look at the mirror.”

Nato is expected to approve Finland and Sweden’s membership requests quickly, but they will not become members before all 30 existing member states ratify their applications, a process expected to take six to 12 months.

A Nato official said that both Finland and Sweden could become formal invitees within “a couple of weeks” before a pivotal Nato summit in Madrid at the end of June.

The official declined to speculate how long ratification could take but added: “We have been hearing from quite a number of allies that they would seek to make this a priority.”

Finland would more than double the border between Nato and Russia thanks to its 1,340km frontier. It would also bring a well-trained military and a high level of preparedness.

The last country to join Nato, North Macedonia, had the longest accession process. The Nato official said: “We weren’t in an extreme security crisis in the Euro-Atlantic region as we are now. These are not normal times.”

Russia has threatened “serious military and political consequences” should either Finland or Sweden join Nato. But Finnish and Swedish officials have been heartened that little Russian mischief has taken place before they take the decision to apply for Nato membership.

Boris Johnson, UK prime minister, on Wednesday signed agreements with both Sweden and Finland to come to their aid with military resources if they were attacked, a move that allayed fears that the countries would be vulnerable during the Nato ratification process.

The Finnish president and prime minister’s joint announcement was hailed in the Baltics. Kaja Kallas, Estonia’s prime minister, wrote on Twitter: “History being made by our northern neighbours…You can count on our full support. We support a rapid accession process. From our side will make necessary steps quickly.”

Gabrielius Landsbergis, Lithuania’s foreign minister, said: “Nato is about to get stronger. Baltics about to get safer.”