Finland is expected to signal on Thursday it is ready to apply for membership of Nato, a day after it and Sweden signed mutual security pacts with the UK.
On Friday, Sweden's ruling Social Democrats are expected to echo that support for alliance membership, 11 weeks after Russia invaded Ukraine.
Wednesday’s new security agreements with the UK – after similar deals signed with the US – are an attempt to address concerns about the so-called “grey zone” both countries will enter after their Nato applications but before accession.
Stockholm and Finland fear their accession – requiring agreement of all 30 existing alliance members – could fall victim to domestic political squabbles, leaving them exposed for an extended period to Russian attacks without full Nato protection.
During a press conference in Helsinki alongside Finnish president Sauli Niinisto, British prime minister Boris Johnson said the UK would come to Finland's assistance, including with military support, in the event of an attack on the country.
Asked if Finland would be provoking Russia by joining Nato, Mr Niinisto said Russian president Vladimir Putin would be to blame for any decision to join the military alliance. "My response would be that you caused this. Look at the mirror," Mr Niinisto said. Speaking after signing the pact, Mr Niinisto said he did not view joining the military alliance as a "zero sum game". "Joining Nato would not be against anybody," he said.
Earlier, Swedish prime minister Magdalena Andersson said: "The prime minister [Boris Johnson] and I have agreed . . . if either country should suffer a disaster or an attack, the United Kingdom and Sweden will assist each other in a variety of ways. The support will be given on request from the affected country, and may include military resources."
She added that the deal would see the UK provide “military resources” if Sweden were attacked, even if Sweden decided not to apply for Nato membership.
After a meeting in Harpsund, the country retreat of Swedish prime ministers, Mr Johnson said the agreements would allow the UK and Sweden “share more intelligence, bolster our military exercises and further our joint development of technology”.
The text of the so-called “Political Declaration of Solidarity” adds that this “intensified co-operation” will remain in line with each country’s existing security and defence policies and will “complement not replace existing European and Euro-Atlantic co-operation”.
A spokesman for Mr Johnson said the UK was ready to increase deployments to the region, "including from the Royal Air Force, British Army and Royal Navy assets and personnel".
Swedish security analyst Malena Britz suggested the Swedish-UK deal goes some way to restoring elements of the EU security and mutual assistance clause in article 42.7 of the Lisbon treaty.
“You could say that we are regaining the support we had from the Brits before they left the EU,” said Ms Britz to Sweden’s TT news wire.
“They discussed supporting with all possible resources, including militarily, and that’s pretty much exactly what is covered in current EU legislation.”
Mr Niinisto is expected to announce on Thursday his stance on joining Nato, two days after parliament’s defence committee called alliance membership “the best solution for Finland’s security”.
“It strengthens Finland’s national defence capability with the support of the union’s significant military resources,” said the committee in a White Paper.
Finland shares a 1,300km border with Russia and its accession to Nato, seen as highly likely, would mark a historic shift in its security policy towards its neighbour. – Additional reporting: Agencies