Nato has welcomed Finland as its 31st member in a flag-raising ceremony at the alliance's headquarters on the outskirts of Brussels. It marks the formal accession of Russia’s western neighbour to the world’s largest military alliance, completing an accelerated application process that started last May, when Finland and neighbouring Sweden abandoned decades of military nonalignment to seek security as Nato members after Russia’s invasion of Ukraine.

Nato’s border with Russia will roughly double with the accession of Finland, which shares a 832-mile (1,340km) border with its eastern neighbour. Moscow has warned it will bolster its defences in the region if necessary.