The UK’s energy watchdog has asked the National Grid for answers after a major power cut affected nearly one million people across England and Wales.
Train passengers were stranded, traffic lights failed to work and thousands of homes were plunged into darkness during the blackout on Friday.
Some train services continue to be disrupted on Saturday morning.
National Grid said it was caused by issues with two power generators and the problem had been quickly resolved.
Regulator Ofgem said it has asked for an “urgent detailed report” to find out what went wrong.
It will then decide what further steps need to be taken, adding: “This could include enforcement action.”
The power outage happened about 17:00 BST on Friday afternoon, National Grid said, with blackouts across the Midlands, the South East, South West, North West and North East of England, and Wales.
The company said power was restored by 18:30 BST.
Northern Powergrid said the problems had affected Newcastle airport and city’s metro system.
And throughout Friday’s evening rush hour there was huge disruption on the railways, as police officers were called in to help travellers and delayed passengers were left stranded for hours.
King’s Cross was among worst-hit stations, with all trains suspended for several hours.
The first train out of the station was at 21:30 to Peterborough, and the first long distance service was packed as it left for Newcastle at 22:46.
Passenger Dayna McAlpine told BBC Radio 5 Live her train took nearly 13 hours to reach London King’s Cross from Edinburgh – a journey which would normally take less than five hours.
“By hour seven things were starting to get pretty tense,” she said. “People were threatening to self-evacuate off the train… Food ran out about five hours ago.”
Others on social media reported having travelled for 12 hours, while some rail passengers were stuck on trains until the early hours of the morning.