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 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.
Some train companies warned the disruption will continue into Saturday morning.
London North Eastern Railway, which runs between King’s Cross and the north of England and Scotland, has cancelled some services.
Thameslink and Great Northern said a number of trains did not end up in their correct location on Friday because of the disruption and problems will continue until 12:00 BST on Saturday.
Eerie journey on the packed 21:00 to Newcastle
By Danny Savage, BBC News correspondent
As virtually every northbound service from London King’s Cross was cancelled last night, one long distance train did manage to get out.
The 21:00 to Newcastle slipped away from Platform 0 at 22:46.
It was eerie onboard, seeing every other platform deserted on a Friday night when there should have been many trains in.
All the doors to King’s Cross were locked when we arrived apart from one.
British Transport Police were letting a handful of people in if they were heading to Peterborough or any stop beyond there to Newcastle.
The train was packed. But slowly those onboard did get to where they wanted to.
Along the way the conversations with strangers were about how acceptable it is for the National Grid to fall short on its commitment and is our power supply really so fragile in 2019?
At the worst of the power cut, around 500,000 people were affected in Western Power Distribution’s area – including 44,500 customers in Wales – while 110,000 Northern Powergrid customers also lost power
In London and the South East 300,000 people were affected, UK Power Networks said, and another 26,000 customers were without power in the North West.
On Friday, Harriet Jackson, described an “apocalyptic” scene on Northcote Road, in Battersea, when traffic lights cut out and cars were not stopping.
And Scott McKenzie, 31, from Cardiff, said “various alarms were going off” during the 15-minute power outage at Newcastle Airport.
“We were literally plunged into darkness and people were using their phones as torches to see and get around,” he added.
At Ipswich Hospital, a back-up generator which was supposed to supply power to outpatient areas did not work after the power cut, causing problems for 15 minutes before power was restored.
In a video posted online, Julian Leslie, the head of national control at the the National Grid, explained the cut was called by “the loss of two generators that connect to the National Grid transmission network in order to export their power”.
“As a result of this, and this rare event, the system needs to protect itself.
“And what was happening as the frequency was falling, the system could see this, and unfortunately the way the system protects itself is by losing some demand.”
On Friday, the Department for Transport, said: “Today’s power outage has had knock on impacts on travel.
“We’re working hard with Network Rail and others to ensure systems are up and running as quickly as possible, so that everyone can complete their journeys safely.”