Finland's formal accession as the 31st member of NATO "has made our Alliance stronger and every one of us safer", Sunak said.

"All NATO members now need to take the steps necessary to admit Sweden too, so we can stand together as one Alliance to defend freedom in Europe and across the world."

Foreign Secretary James Cleverly, attending the NATO meeting in Brussels where Finland acceded, announced another pound12 million ($16 million) in "non-lethal" military aid to Ukraine such as medical equipment and rations.

"Russia thought its aggression would divide us. Instead, we are bound tighter together, resolute in our defence of the principles of freedom and the rule of law," Cleverly said.

"Let us be clear that our door remains open. We will welcome further allies with open arms and we continue to push for Sweden's swift accession."