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.

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.

A massive power cut hit large parts of Spain and Portugal on Monday, causing widespread chaos and disruption. The outage left millions of people without electricity, while internet and phone networks were also down. Trains were cancelled and evacuated in some regions, traffic lights stopped working amid mounting delays at airports.