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.

Several airports, including Madrid airport, which is Spain's busiest, had no electricity, El Mundo reported. Major cities including Barcelona, Seville and Valencia were without power. Parts of France were also affected by power loss.

Traffic lights went dark, mobile phone coverage cut out and the transport industry ground to a halt in tourist cities such as Barcelona, Madrid and Lisbon. The cause of the outage remains unclear. As power flickers back, more than 40 flights have been cancelled from Spain and Portugal’s major airports.