"If Sweden were attacked and looked to us for support then we would provide it," Johnson told reporters at a joint press conference in Sweden.

Under the Political Declaration of Solidarity, "if either country should suffer a disaster or an attack, the UK and Sweden will assist each other in a variety of ways... and may include military resources," Andersson added.

Johnson is expected to sign a similar agreement in Helsinki later Wednesday, according to his office.

Sweden and Finland are to decide in the coming days whether to ditch decades of military non-alignment and join NATO, following Russia's invasion of Ukraine.

Wary of sparking Moscow's ire as it opposes any NATO expansion, the two Nordic countries have been seeking security assurances from NATO members that they would be protected during the interim period between submitting an application and obtaining full membership.

Helsinki and Stockholm have already held talks with powerful NATO members, including the US, France and Germany, but the document signed with Britain is the first agreement that has been made public.