Energy watchdog Ofgem has demanded an urgent report from National Grid after a major power cut caused travel chaos and cut electricity for almost 1 million people in England and Wales.

More than 900,000 customers have had their power restored following blackouts late on Friday afternoon after what the National Grid Electricity System Operator said were issues with two generators.

The power cut stopped traffic lights from working, plunged Newcastle Airport into darkness, affected Ipswich Hospital and caused huge disruption on the railways during the busy Friday night commute.

"In any incident the priority is to get power restored to customers as quickly as possible," Ofgem said in a statement, adding that it understood "the frustration this power cut has caused consumers."

The energy watchdog said it had asked for "an urgent detailed report from National Grid so we can understand what went wrong and decide what further steps need to be taken. This could include enforcement action."

Major power outage causes chaos across England and Wales Show all 10 1 /10 Major power outage causes chaos across England and Wales Major power outage causes chaos across England and Wales People waiting outside of King's Cross station, London, as all services in and out of the station were suspended. A large power cut caused apocalyptic rush-hour scenes across England and Wales, with traffic lights down and trains coming to a standstill PA Major power outage causes chaos across England and Wales People waiting inside King's Cross station, London. Nearly a million customers faced blackouts late on Friday afternoon after what the National Grid Electricity System Operator said were issues with two generators, which are now resolved PA Major power outage causes chaos across England and Wales People walking in complete darkness at Clapham Junction station in London PA Major power outage causes chaos across England and Wales A LNER staff member pushes a trolley of bottled water at Peterborough station, as passengers wait for news during travel disruption on the East Coast mainline PA Major power outage causes chaos across England and Wales People walking in complete darkness at Clapham Junction station in London PA Major power outage causes chaos across England and Wales A ticket machine out of order at Clapham Junction station PA Major power outage causes chaos across England and Wales People walking through Clapham Junction station PA Major power outage causes chaos across England and Wales People walking in complete darkness at Clapham Junction station PA Major power outage causes chaos across England and Wales An empty shop in darkness at Clapham Junction station PA Major power outage causes chaos across England and Wales Temporary lighting is set up during a power cut affecting Clapham Junction train station PA

Along with the travel chaos, a back-up generator failed at Ipswich Hospital after the power cut, a spokeswoman for East Suffolk and North Essex NHS Foundation Trust.

The generator which was supposed to supply power to outpatient areas of the hospital did not work as expected.

"There were some issues with regard to our outpatient areas and the generator that provides cover (to them)," she said, adding that other generators "kicked in as was required" and "patients were kept safe and cared for throughout".

The problems lasted about 15 minutes before power was fully restored, she said.

Professor Tim Green, co-director of the Energy Futures Laboratory, Imperial College London, believes the two generators disconnected were at Little Barford and Hornsea.

He said: "The first generator to disconnect was a gas fired plant at Little Barford at 16:58. Two minutes later Hornsea Offshore wind farm seems to have disconnected. This might be linked to disturbance caused by first generator failing; might not. We will need to wait for National Grid's full technical investigation to get to bottom of that."

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 the North West of England, Electricity North West said.

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Problems with power were first detected late on Friday afternoon when UK Power Networks, which controls power lines for London and the South East, and Western Power Distribution in the Midlands, the South West and Wales both confirmed widespread outages.

UK Power Networks tweeted on Friday evening: "We're aware of a power cut affecting large parts of London and South East. We believe this is due to a failure on National Grid's network, which is affecting our customers."