During a ceremony in Brussels, Finland has formally joined NATO as its 31st member.
The Nordic country’s border with Russia has doubled as a result of its formal entry into the largest security alliance in the world.
After Russia invaded Ukraine in 2022, Finland and Sweden together applied to join Nato.
Although Sweden are still awaiting approval, Finland’s membership represents a major change in Europe’s security landscape.
The country adopted neutrality after its defeat by the Soviets in the Second World War, but its leaders signalled they wanted to join the alliance just months after Russian president Vladimir Putin’s invasion of Ukraine sent fear through Moscow’s neighbours.
The move is a strategic and political blow to Mr Putin, who has long complained about Nato’s expansion toward Russia and partly used that as a justification for the invasion. The alliance says it poses no threat to Moscow.
Finland shares an 832 mile land border with Russia- the largest in Europe.
The process was formalised this afternoon when Finnish foreign minister Pekka Haavisto handed over signed accession documents to US Secretary of State Antony Blinken.
Following their accession, the length of Russia’s border with Nato member states has now doubled.
Rishi Sunak has welcomed Finland becoming the newest member of the Nato defence alliance.
The Prime Minister said: ‘This is an historic day for Finland and for Nato. Their accession has made our alliance stronger and every one of us safer.
‘All Nato members now need to take the steps necessary to admit Sweden too, so we can stand together as one alliance to defend freedom in Europe and across the world.’
Russia warned that it would be forced to take ‘retaliatory measures’ to address what it called security threats created by Finland’s membership.
It has also warned it will bolster forces near Finland if Nato sends any additional troops or equipment to its 31st member country.
Earlier today, Russian defence minister Segei Shoigu announced that a number Belarusian jets had been fitted to carry nuclear warheads in response.
Nato secretary-general Jens Stoltenberg told reporters at the alliance’s headquarters in Brussels that ‘there will be no Nato troops in Finland without the consent of Finland’.
But he refused to rule out the possibility of holding more military exercises there and said that Nato would not allow Russia’s demands to dictate the organisation’s decisions.
‘We are constantly assessing our posture, our presence. We have more exercises, we have more presence, also in the Nordic area,’ he said.
Kremlin spokesman Dmitry Peskov said on Tuesday that Finland’s membership reflects the alliance’s anti-Russian course and warned that Moscow will respond depending on what weapons Nato allies place there.
‘We will closely monitor what will be going on in Finland and how Nato will use the territory of Finland for the deployment of weapons, equipment and infrastructure next to our border that would potentially threaten us. Measures will be taken dependent on that,’ Mr Peskov said in a conference call with reporters.
But Mr Peskov also sought to play down the impact, noting that Russia has no territorial disputes with Finland.
It is not clear what additional military resources Russia could send to the Finnish border. Moscow has deployed the bulk of its most capable military units to Ukraine.
Mr Stoltenberg said that Finland will benefit from Nato’s ‘iron-clad security guarantee” under which all member countries vow to come to the defence of any ally that comes under attack.
‘By (Finland) become a full-fledged member, we are removing the room for miscalculation in Moscow about Nato’s readiness to protect Finland, and that makes Finland safer and stronger, and all of us safer,’ Mr Stoltenberg said.