Finland and Sweden are expected to announce this week whether they will apply to join the alliance, after decades of non-alignment.

Both Helsinki and Stockholm have been rattled by Moscow's invasion of its pro-Western neighbour, prompting public support for a reversal in policy.

Johnson's official spokesman told reporters that the prime minister would visit both countries on Wednesday for discussions on Ukraine and also wider European security.

"We support countries' democratic capability to decide on things like NATO membership," he added.

"We understand the positions of Sweden and Finland and that is why the prime minister is going to discuss these broader security issues."