THE cause of massive power cuts across Spain and Portugal remained a mystery today, after one of Europe’s worst-ever blackouts grounded flights, paralysed Metro systems, disrupted mobile phone networks and shut down cash machines.

Though some isolated disruption remained, more than 99 per cent of Spain’s energy demand was being met by 7am, national power supplier Red Electrica said.

In Portugal, grid operator REN said all 89 power substations were back online and power had been restored to all 6.4 million customers.

As life began to return to normal, Spanish Prime Minister Pedro Sanchez said that his government's priorities were to restore the electricity system and find the causes of the blackout so that a similar event “never takes place again.”

His country lost 15 gigawatts, equivalent to 60 per cent of its national demand, in just five seconds.

Red Electrica head of system operations Eduardo Prieto noted two steep, back-to-back “disconnection events” before Monday’s blackout, telling a press conference today that more investigation was needed to understand why they took place.

Suggestions that an unusual weather event or a cyber attack were to blame have been dismissed by the relevant agencies.