Finland has become Nato’s newest member with the UK saying the move demonstrates the strength of the alliance in the face of Vladimir Putin’s aggression.
Prime Minister Rishi Sunak said Finnish membership had made the alliance stronger and pushed for Sweden to also be given permission to join the group.
Foreign Secretary James Cleverly said Russia’s invasion of Ukraine had resulted in a Nato that was larger and “bound tighter together”.
Russia’s invasion of Ukraine led the previously neutral Finland and Sweden to seek membership.
Finland, which has an 830-mile border with Russia, formally became the 31st member of the alliance at a ceremony at Nato’s Brussels headquarters on Tuesday.
Neighbouring Sweden has seen its process of joining the organsation delayed by objections from Turkey and Hungary, but Mr Sunak made clear he wanted those issues resolved.
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.”
Mr Cleverly, who was at Nato’s headquarters, said: “Today we see, as a direct result of Vladimir Putin’s aggression and his illegal invasion of Ukraine, the day where a new ally joins our defensive alliance.”
He added: “Russia thought its aggression would divide us. Instead, we are bound tighter together, resolute in our defence of the principles of freedom and the rule of law.
“Let us be clear that our door remains open.
“We will welcome further Allies with open arms and we continue to push for Sweden’s swift accession.”
Defence Secretary Ben Wallace said: “Let Finland be a lesson to President Putin.
“Finland chose to join, based on their own free will.
“The freedom to choose their alliances as a sovereign state is a matter for their citizens and their citizens alone.”
Kremlin spokesman Dmitry Peskov warned that Moscow will respond depending on what weapons Nato allies place in Finland.
“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.
Meanwhile the UK announced £10 million for Nato’s support package for Ukraine, providing non-lethal aid such as medical equipment, rations, fuel and counter-drone equipment.
Some £2 million will also go to Nato efforts to help Georgia, Moldova and Bosnia & Herzegovina defend themselves against Russian activity.
