Power blackouts that left millions of people across Spain and Portugal without electricity may have been caused by a bizarre atmospheric phenomenon, though the true cause is yet to be determined.

Spain and Portugal have regained access to electricity after one of Europe’s worst blackouts paralysed transport systems, disrupted mobile communications and postponed medical procedures.

Power has been restored to tens of millions of people across almost all of Spain and Portugal, but disruptions to transport services were expected to stretch into a second day as questions mount over what caused a massive blackout.