Electricity has returned to most of Spain and Portugal but with continued disruptions on the second day after an unknown failure caused a massive blackout across much of western Europe.

Red Eléctrica, Spain’s electricity grid operator, ruled out a cyberattack, human error or unusual weather. Experts say it could take weeks to determine the cause.

The hunt has begun for the cause of a massive blackout across Spain, Portugal and parts of France that halted trains, bank machines and traffic lights, in one of Europe's biggest ever power system collapses.