An inexplicable power outage hit Spain and Portugal on Monday, knocking out traffic lights, causing congestion and disrupting airport operations and other means of transport.

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.

The massive power cut has left areas across Spain and Portugal facing gridlock as trains and planes cancelled and people told not to travel