UK Prime Minister Boris Johnson is visiting Sweden and Finland on Wednesday to sign the relevant agreements.
"The Prime Minister is signing historic declarations with Sweden and Finland today to reinforce their security and fortify northern Europe's defences, in the face of renewed threats," the prime minister's office said in a statement.
The Downing Street said Johnson has already signed mutual security assurances with Swedish Prime Minister Andersson and will now travel to Helsinki to sign a similar security declaration with Finnish President Sauli Niinisto.
The documents entail "mutual security assurances with the UK to tackle traditional, hybrid and cyber threats," the statement read.
In particular, the declarations provide for gradual changes in defense and security cooperation between the UK and the two countries, increasing the sharing of intelligence and joint military operations, and strengthening the overall security in northern Europe.
"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," Johnson is quoted as saying in the statement.
According to Johnson's office, by signing security declarations with Finland and Sweden, the UK also pledged to support the two countries' armed forces in the event of a crisis or attack by other nations.
At the same time, the signatory countries said in joint statements that the declarations are political in nature and are not legally binding under international law.