British Prime Minister Boris Johnson has said he had agreed new deals with Sweden and Finland to bolster European security, pledging to support both countries' armed forces should they come under attack.
Mr Johnson signed the new declaration with Sweden, described by Britain as "a step-change in defence and security cooperation", during a visit to Stockholm.
He signed a similar agreement in Helsinki this afternoon.
"What we are saying, emphatically, is that in the event of a disaster or the event of an attack upon Sweden, then the UK would come to the assistance of Sweden with whatever Sweden requested," Mr Johnson said after meeting Swedish Prime Minister Magdalena Andersson.
Russia's invasion of Ukraine has forced a rethink of how Sweden and neighbour Finland safeguard national security.
Both are expected to join NATO, but both are worried they would be vulnerable while their applications are processed, which could take up to a year.
"In times of crisis, cooperation becomes even more important," Ms Andersson told reporters at a briefing with Mr Johnson at the Swedish government's country retreat south of Stockholm.
"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 by the affected country and may include military resources."
Sweden and Finland have also received assurances of support from the United States and Germany.
The British statement said the new arrangements would intensify intelligence sharing and accelerate joint military training, exercises and deployments.
Sweden and Finland already have close ties with Britain and are part of the Joint Expeditionary Force, a multinational military group focused on security in the High North, North Atlantic and Baltic Sea regions.
But the war in Ukraine has increased fears in Stockholm and Helsinki that bilateral agreements are no substitute for Article5 of NATO's charter, which guarantees that an attack on one member state would be considered as an attack on all.
Sweden is coordinating with Finland over possible NATO membership and both countries are expected to announce their intentions in the coming days.