The massive power cut that plunged huge swathes of Spain and Portugal into chaos earlier this year was caused by a miscalculation, the Spanish government has said.

Airports, trains and internet were among the services affected by the blackout in late April, with traffic lights also down - causing huge tailbacks.

Millions of people were affected and parts of the two countries were brought to a standstill as authorities raced to find the source of the problem.

There was discussion about whether a cyberattack was to blame, but today Spain's energy minister said the issue was a miscalculation by the Spanish power grid operator REE.

Sara Aagesen said REE did not have enough thermal power stations switched on during peak hours of April 28 when the surge caused a chain reaction leading to the power outage.

"The system did not have sufficient dynamic voltage control capacity," she said.

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The government report, which was due to be made public on Tuesday, found that some of the power plants required by law to regulate the grid's voltage failed to do so.

Electricity grids in Europe are kept at 50 Hertz frequency in order to maintain stability - and even a slight deviation can lead to damage.

Power plants "should have controlled voltage and, moreover, many of them were economically remunerated to do so. They did not absorb all the reactive power that was expected in a context of high voltages," Ms Aagesen said.

The investigation found no evidence of a cyberattack, she added.