Spain and Portugal have been hit by a widespread power outage on Monday, leaving millions without electricity, according to reports.

In Portugal, official sources told domestic media that the outage, which hit around late morning local time, was nationwide, while similar reports have emerged from Spain.

Madrid's Barajas International Airport was left without power, and telecommunications have also been affected, with residents across the two countries saying they have no access to mobile networks. Meanwhile, other airports have come to a standstill throughout the region.

Numerous passengers have been left stranded in the metro systems of the Spanish and Portuguese capitals, with trains stuck in tunnels between stations, Euronews Portugal reports. The blackout has affected hospitals, too, including Madrid's La Paz and multiple medical facilities in Portugal.

The Spanish government has gathered for an emergency session at Moncloa and is monitoring the situation as it develops, according to our colleagues at Euronews Spain.

Citizens have been asked not to dial 112 unless in a genuine emergency to avoid overloading the lines.

Residents of Andorra and parts of France bordering Spain were also reporting being hit by the blackout. Further outages have been reported as far as Belgium, according to the latest information.

The cause is yet unclear. Endesa and Iberdrola, Spain's two major electric companies, Endesa and Iberdrola, are investigating the incident.

Domestic media are reporting issues with the European electric grid, which affected national grids in the Iberian Peninsula, however.

A fire in the south-west of France, on the Alaric mountain, which damaged a high-voltage power line between Perpignan and eastern Narbonne, has also been identified as a possible cause, Portugal's national electric company REN said.

This is a developing story and our journalists are working on further updates.