Agencies Madrid

Power was almost fully restored to Spain and Portugal on Tuesday although many questions remained about what caused one of Europe's most severe blackouts that grounded flights, paralysed metro systems, disrupted mobile communications and shut down ATMs across the Iberian Peninsula.

By 7 am, more than 99 per cent of energy demand in Spain had been restored, the country's electricity operator Red Electrica said. Portuguese grid operator REN said Tuesday morning all of the 89 power substations were back online since late last night and power had been restored to all 6.4 million customers.

By Tuesday morning, life was returning to normal: schools and offices reopened in Spain, traffic eased along the capital's main arteries and public transport restarted after significant delays. Spanish authorities did not provide new explanations for what caused the blackout, one of the most serious to ever take place in Europe.

In a televised address Monday night, Spanish Prime Minister Pedro Sanchez said the power grid for the Southern European nation of 49 million people lost 15 gigawatts - equivalent to 60 per cent of its national demand - in just five seconds.