Boris Johnson will sign historic security assurance declarations with Sweden and Finland in the face of Russia's invasion of Ukraine, pledging to "bolster military ties" and support both countries should they come under attack.
The Prime Minister arrived in Stockholm on Wednesday before travelling to Harpsund, the country residence of his Swedish counterpart, Magdalena Andersson, where he set out a UK commitment to come to the country's aid in the event of a crisis
An offer to increase deployments to the region, including with Royal Air Force, British Army and Royal Navy personnel and assets, will also be made.
Mr Johnson is also set to visit to Finland, where he is expected to formalise a similar agreement with the country's President Sauli Niinisto.
It comes as both countries consider the prospect of NATO membership in the face of Russian President Vladimir Putin's ongoing military aggression.
The Prime Minister will look to make clear the UK's unwavering support for NATO's open-door policy during the visit.
Mr Johnson said: "We are steadfast and unequivocal in our support to both Sweden and Finland and the signing of these security declarations is a symbol of the everlasting assurance between our nations.
"These are not a short-term stop-gap, but a long-term commitment to bolster military ties and global stability, and fortify Europe’s defences for generations to come."
The declarations build on claims made earlier in the month that the UK would always aid Finland if it were attacked by Russia, regardless of whether the country was a member of NATO.
Watch: The Defence Secretary said this month that the UK will stand with Finland against any Russian threats.
Defence Secretary Ben Wallace said it was "inconceivable" that Britain would not help either Finland or Sweden if they were in crisis, even "without any big formal agreement".
The declarations signed today will allow the UK to co-operate with key Nordic partners and their armed forces, in all domains, including cyberspace. They will also allow for closer collaboration on new technology and intelligence gathering.
Mr Johnson held talks with Ms Andersson and Mr Niinisto in March as part of a meeting of the Joint Expeditionary Force nations, which includes Denmark, Estonia, Iceland, Latvia, Lithuania the Netherlands and Norway.
After the meeting, Downing Street said the two leaders agreed that "Putin's invasion had dramatically changed the landscape of European security".
Finland shares a lengthy land border with Russia and is only about 250 miles from St Petersburg.