A statement by National Grid on Friday night said the issue had been resolved, though train services across the country were widely disrupted, with carriages being stranded in some areas.
Transport for London tweeted: "Due to a large scale National Grid failure there is a power cut in the London and South East areas, meaning that some traffic lights are down".
Delays and cancellations continued Saturday at London's King's Cross, one of the country's busiest train stations.
The hour-long outage on Friday evening left nearly 1 million homes without power while two of London's busiest train stations closed at rush-hour because of overcrowding as services were canceled or delayed.
BBC also quoted Network Rail as saying that trains across large parts of England were stopped after a "power surge on the National Grid" but its signaling system has now come back online.
The generator which was supposed to supply power to outpatient areas of the hospital did not work as expected.
A Government spokesman said the issue is "now resolved".
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The first train bound for Peterborough left the central London station at around 9.30pm after all services were halted for several hours.
The power outage has caused widespread traffic disruption on the roads and railways.
"No one knew what was going on and, given it's a Friday afternoon, it's the last thing you want to encounter".
Meanwhile, Western Power Distribution (WMD) also tweeted: "We are now experiencing a major incident on the national electricity infrastructure".
National Grid, which runs Britain's power network, said on Saturday it was confident of "no malicious intent" behind a major outage Friday that hit almost one million people and caused travel chaos.
Previously, the UK Power Networks said on twitter that large parts of London and the South East are without electricity.
Western Power Distribution shared a similar message, and said they were in the process of restoring power to customers. We believe this is due to a failure to National Grid's network, which is affecting our customers.