A power surge that caused a widespread blackout in Spain and Portugal was the 'most severe' in Europe in the last 20 years, and the first of its kind, a report has found.

Damian Cortinas, president of the association of electricity grid operators Entso-e, said the incident was the first known blackout to be caused by overvoltage, which occurs when there is too much electrical voltage in a network.

"This is new territory," Cortinas said, adding the role of Entso-e was "not to apportion blame to any party" over the cause.

April's outage caused significant disruption for nearly a day when it plunged areas into darkness, cutting internet and telephone connections and halting transport links.

The blackout affected large parts of Spain and Portugal and briefly impacted southwestern France.

The report, released on Friday, focused on the condition of the power systems on the day of the outage and the sequence of events that led up to it. A series of 'cascading overvoltages'—an increase in the electrical supply voltage above the established norm—was behind the outage, it concluded.

Overvoltage can be triggered by surges in networks due to oversupply or lightning strikes, or when protective equipment is not sufficiently robust.

According to the report, automatic defense plans were activated but could not prevent the power system from shutting down. It follows multiple investigations by the Spanish government, power, and grid companies, as well as separate probes by the national energy watchdog and Spanish lawmakers.

The Spanish government believes the Entso-e report supports its own findings, with Ministers concluding that both the national grid provider Red Eléctrica and private electricity companies were at fault. However, both companies argue that poor planning among grid operators caused the issue.

The outage triggered a political debate about Spain's energy model. The opposition argued that the growing reliance on renewable energy could have contributed to the blackout, while the government dismissed these claims.

The loss of power also forced organizers of the Madrid Open Tennis to stop a match, as millions experienced disrupted lives; traffic lights ceased functioning, buildings went dark, and hospitals activated emergency procedures.