The energy watchdog, Ofgem, is demanding answers from the National Grid after a power cut left people stuck in trains for up to nine hours and cut electricity to almost 1 million people in England and Wales.

The biggest power outage in a decade caused widespread disruption on the rail network during the evening rush hour on Friday. Traffic light systems stopped working, causing gridlock in some areas, and Newcastle airport was left in darkness. Power had been restored to 900,000 customers by Saturday, but the rail network was struggling to get services back to normal.

Facebook Twitter Pinterest People wait inside Kings Cross station, where all train services were suspended for several hours. Photograph: Lewis Pennock/PA

In a statement released on Saturday, the National Grid acknowledged the disruption that had been caused and said it was investigating what happened.

Ofgem had earlier demanded an “urgent detailed report” from the National Grid to better understand what went wrong, and threatened enforcement action.

A National Grid spokesperson said: “The root cause of yesterday’s issue was not with our system but was a rare and unusual event, the almost simultaneous loss of two large generators, one gas and one offshore wind, at 4.54pm. We are still working with the generators to understand what caused the generation to be lost.”

Speaking on the BBC Radio 4 Today programme, the National Grid spokesman Duncan Burt said what happened was an “incredibly rare event”.

Scientists have said the blackout was not caused by the use of wind generation in the power network. They believe the disconnection of Little Barford gas-fired power station, in Bedfordshire, and Hornsea offshore wind farm, off the coast of Yorkshire, triggered the outage.



Prof Tim Green, the co-director of the Energy Futures Laboratory, at Imperial College London, said Little Barford was the first generator to disconnect, followed bythe Hornsea disconnection two minutes later.

“This would seem to be a technical failure [or] error,” he said. “[It] might be linked to disturbance caused by [the] first generator failing – might not. We will need to wait for National Grid’s full technical investigation to get to the bottom of that.”



He said the incident did “not appear to be due to wind generation reducing owing to reduced wind speed”.



The outage has been compared to a similar blackout in May 2008 when the Sizewell B nuclear plant in Suffolk and the Longannet coal-fired station in Fife went offline within minutes of each other.

Friday’s power outage even affected hospitals. A back-up generator at Ipswich hospital which was supposed to supply power to outpatient areas did not work as expected.

A spokeswoman for East Suffolk and North Essex NHS foundation trust said other generators worked as required and “patients were kept safe and cared for throughout” the 15-minute period before power was restored.

Facebook Twitter Pinterest Temporary lighting guides passengers through Clapham Junction train station in London. Photograph: Yui Mok/PA

A transport union leader said the government must be held to account for the power cut.

Manuel Cortes, the general secretary of the Transport Salaried Staffs Association, said: “We urgently need answers over this fiasco. Having our rail network brought to a standstill in this way is totally unacceptable.

“As we face the growing prospect of a no-deal Brexit it’s reasonable to wonder if this is a foretaste of things to come. Along with an economy sliding towards recession and expected food shortages, we now seem to be a country where blackouts happen without warning, travel grinds to a halt, traffic lights stop working and – terrifyingly – hospitals are left without power.”

He added: “Boris Johnson can’t remain silent over this – he must quickly provide answers and illumination.”

Around 300,000 UK Power Networks customers were affected in London and the south-east, a spokeswoman said, and Western Power Distribution said around 500,000 people were affected in the Midlands, south-west and Wales, with power restored to them all shortly after 6pm.

A spokeswoman for Northern Powergrid, which serves Yorkshire and the north-east, said 110,000 of its customers lost power, while at least 26,000 people were without power in north-west England, Electricity North West said.

Facebook Twitter Pinterest An LNER staff member provides bottled water for passengers stranded at Peterborough station. Photograph: Martin Keene/PA

Trains began to run out of Kings Cross late on Friday night after the station was shut down for several hours. Passengers were filmed forcing their way through the barriers in an attempt to board the first northbound service, a train bound for Peterborough at around 9.30pm.

Nick King, the network services director for Network Rail, said it apologised on behalf of the rail industry for the disruption faced by passengers.

“Unfortunately one particular fleet of trains had a major systems failure as a result of the National Grid failure,” he said. “Many of these trains were unable to restart on their own and had to be attended by an engineer and this caused significant disruption across parts of the network.

“We worked flat out with train operator colleagues and the British Transport police to safely get passengers off impacted trains.”