Finland signed joint security declarations with Britain on Wednesday, a move that president Sauli Niinistö described as a "big step forward" for co-operation between the two countries.
The British government said that the bilateral agreements made with both Finland and Sweden would "reinforce their security and fortify northern Europe’s defences, in the face of renewed threats".
British PM Boris Johnson flew to Stockholm and then Helsinki on Wednesday to discuss security policy with the leaders of the two Nordic countries.
Johnson said that Russia's war against Ukraine has ended the post-Cold War era and "reshaped our future" as countries that have previously avoided military alignment now rethink their foreign policy.
"We signed a joint statement. We will stand together and support each other in any circumstances, in good and bad weather," president Niinistö said in a press conference following the two leaders' meeting.
When asked about the Kremlin's thoughts on a possible Nato membership Niinistö denounced Russia's attempt to remove Finland and Sweden's agency over the decision.
"If the case is that we join well, my response [to Russia] would be that you caused this, look at the mirror," Niinistö said.
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Johnson's statement of solidarity follows several other allies who have offered support in recent weeks, as both Finland and Sweden close in on decisions over whether or not to join Nato.
While in Sweden, UK Prime Minister Boris Johnson declared that Britain would defend the country if it were attacked. He added that the UK would support Finland's decision on Nato membership, whatever it may be.
The issue has shot up the political agenda in recent weeks, with public support for a Nato application increasing in both countries, after Russia invaded Ukraine in February.
Finland is expected to announce its decision in the coming days, with President Niinistö and Prime Minister Sanna Marin (SDP) due to give their own views on Nato at 10 am on Thursday. Parliamentary parties will then meet and discuss their positions, with statements expected to follow over the weekend.
Estonian president: Russia unlikely to attack Finland in case of Nato application
Estonian President Alar Karis was also among the leaders to discuss the Nato decision with Niinistö on Wednesday.
"For Estonia, and not just Estonia but the rest of the Nato members, this is very good news if Finland and Sweden join Nato," Karis told an Yle reporter after his meeting with the Finnish president, adding that Russia was unlikely to attack Finland militarily in the aftermath of a membership application.
"Russia has given a number of threats so it is probably going to say something but it is not that important. I don't think that Russia will enter somehow millitary to Finland and other countries," Karis said.
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