Electricity has been restored to almost 90 percent of mainland Spain after a massive blackout in the Iberian Peninsula on Monday.

According to the national electricity grid operator Red Electrica de Espana (REE), 87.37 percent of electricity supply had been restored by 4am on Tuesday.

On Monday evening, Prime Minister Pedro Sanchez had announced that half of Spain's electricity supply had been restored.

“The challenge is to continue restoring power in the coming hours,” he had added in a televised address.

The prime minister, however, did not specify when power would be fully restored, and said some workers would have to stay home on Tuesday.

The exact cause of this widespread power outages in Spain and Portugal remains unknown. Portuguese Prime Minister Luis Montenegro said the source of this blackout was “probably in Spain,” while Spanish premier Pedro Sanchez said authorities were investigating the cause.

 “All possible causes are being analyzed, I emphasize that no theory, no possibility is excluded,” he added.

Portuguese authorities had earlier said the blackout could have been caused by a rare atmospheric phenomenon linked to temperature fluctuations.