Large swathes of the country were hit overnight by high winds and heavy rain, threatening further travel disruption through the weekend.
London: National Grid, which runs Britain's power network, on Saturday, said it was confident of "no malicious intent" behind a major outage Friday that hit nearly one million people and caused travel chaos.
The company has blamed the blackouts, that left more than 900,000 customers without power for several hours and transport systems reeling, on the loss of two generators in quick succession.
It has vowed to "learn any lessons" from what it described as an "unexpected and unusual event", which saw evening commuters stranded on trains, traffic lights failing and homes plunged into darkness.
"We are already very confident that there was no malicious intent or cyber attack involved," National Grid director of operations Duncan Burt told the BBC.
"This will require careful study to make sure that we do learn any lessons that come out of it and that the next time this happens disruption is minimised."
He said the company would provide a technical report on the outage to industry regulator Ofgem, which on Friday demanded an immediate investigation into the incident.
"Ofgem has 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," the regulator said in a statement.
"This could include enforcement action."
Industry experts believe the blackout was prompted by a gas-fired power station in Bedfordshire, north of London, and then an offshore wind farm in Yorkshire, northern England, both disconnecting from the grid.
The outage lasted for several hours, affecting around 300,000 customers in London and the southeast, and 500,000 in the Midlands, southwest England and Wales, according to regional power utilities.
Some 110,000 were affected in Yorkshire and northeast England.
It sparked rush-hour transportation misery, as some traffic lights faltered on the capital's streets and air traffic was disrupted at the airport in Newcastle.
"All the traffic lights were down, but there were no police present... it was like witnessing something out of an apocalyptic film," Harriet Jackson told Britain's Press Association in south London.
Many trains were delayed or cancelled on Britain's national rail network, with efforts to return services to normal hampered by severe weather on Saturday.
Large swathes of the country were hit overnight by high winds and torrential rain, threatening further travel disruption through the weekend.
Keith Bell, a professor of electronic and electrical engineering at the University of Strathclyde, said the power cut was a "relatively small event" compared to blackouts seen elsewhere around the world.
"This event is perhaps a reminder that, even though we have never suffered a whole system blackout in Britain... restoration plans always need to be kept under review," he added.