Spain and Portugal were hit by a widespread power outage on Monday that paralysed public transport, caused large traffic jams and delayed flights.

Utility operators sought to restore the grid but Spanish electricity transmission operator Red Eléctrica said the outage, the cause of which was not immediately known, could last from six to 10 hours. Officials said the possibility that it was caused by a cyber attack had not been ruled out.

The outage caused chaos in parts of Portugal and Spain as traffic lights stopped working, causing gridlock. Transport networks were halted, hospitals were left without power and people were trapped in the metro and in elevators.