Johnson is due to sign the new declarations, described by Britain as “a step-change in defence and security cooperation”, during visits to both Sweden and Finland on Wednesday.

“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,” Johnson said in a statement.

The government statement added: “The Prime Minister will set out the UK’s intention to support the two nations’ armed forces should either face crisis or come under attack.”

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. 

Sweden has also received assurances of support from the U.S. and Germany. 

The British statement said the new arrangements would intensify intelligence sharing, and accelerate joint military training, exercises and deployments.

(Reporting by William James and Simon Johnson, Editing by Kylie MacLellan)