Duncan Burt, director of operations for the National Grid Electricity System Operator, said the “incredibly rare event” was triggered by two power stations – gas and then wind – disconnecting near-simultaneously on Friday.
Questioned on Radio 4’s Today programme on whether wind turbines may have cut out in stormy weather and gales, Mr Burt said that was not the case.
“The events we saw yesterday really have nothing to do with changes in wind speed or the variability of wind,” he added.
“There was severe weather on the network [but] we routinely operate the grid now at very high levels of wind power.”
Energy watchdog Ofgem has demanded an urgent report on the mass outage, which stopped traffic lights from working, plunged Newcastle Airport into darkness, affected Ipswich Hospital and caused huge disruption on the railways – including closing London King's Cross and St Pancras – during Friday’s evening rush hour.
One passenger said her train took nearly 13 hours to reach London from Edinburgh - a journey which would normally take less than five hours.
“By hour seven things were starting to get pretty tense,” Dayna McAlpine told BBC Radio 5 Live. “We were being held in the middle of nowhere, the food ran out.”
Train cancellations and delayed continued on some lines on Saturday morning, as high winds started to bring trees down on railway tracks and cause new disruption.
King's Cross reopened late on Friday night after the station was shut down for hours amid “apocalyptic” rush-hour scenes.
Passengers were filmed forcing their ways through the barriers in an attempt to get themselves on to the first northbound service, while traffic light outages caused chaos on the roads.
Harriet Jackson described an “apocalyptic” scene when she witnessed the power outage causing traffic lights to cut out after leaving Clapham Junction railway station in south London at around 5pm.
“All the traffic lights were down, but there were no police present, which meant it was dangerous to cross - cars weren't stopping either,” the 26-year-old told Press Association.
“It was like witnessing something out of an apocalyptic film.
“No one knew what was going on and, given it's a Friday afternoon, it's the last thing you want to encounter.”
Newcastle Airport was plunged into darkness and Network Rail said train signals had lost power over a large area including Newport, Gloucester, Ashford, Bristol, Eastbourne, Hastings, Three Bridges and Exeter.
“All trains were stopped while our back-up signalling system started up,” a spokesperson added.
Mr Burt said the loss of two major generators caused automatic systems to kick in and disconnect parts of England and Wales in order to preserve energy supply elsewhere.
“We can see from our systems that lots of other generators moved in to secure the grid but the loss from those two generators is larger than our standards would routinely secure for so that took the grid to a place which meant we needed our secondary backup safety systems in,” he added.
“That doesn’t take away from the very significant impacts from this rare and very unusual event and we really appreciate the large scale of disruption caused to travellers around the country.”
David Hunter, an energy analyst at Schneider Electric, said the scale of disruption should act as a “wake-up call” for National Grid, as well as for businesses, hospitals and critical national infrastructure to ensure they have fail-safes in place.
“It raises questions in that although this was a very rare event, it was very significant,” he told the Today programme.
“National Grid must ensure that its processes for very quick frequency response and backup power generation are being operated exactly as they should.”
He said there was a “very high percentage” of wind generation on Friday, and that it was not as effective at absorbing sudden fluctuations in frequency as gas, coal and nuclear power.
Mr Hunter added: “The growing wind part of the energy mix creates challenges that the National Grid must demonstrate it can meet.”
Professor Tim Green, co-director of the Energy Futures Laboratory at Imperial College London, believes the two generators disconnected were at Little Barford, in Bedfordshire, and Hornsea, Yorkshire.
He said: “The first generator to disconnect was a gas fired plant at Little Barford at 4.58pm. 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.”
Mr Burt said technicians were investigating what caused the generators to go down, and whether automatic systems are selecting the right areas to cut off in such events.
He denied that human error was to blame and described the situation as an “exceptional event” on a scale not seen in more than a decade.
“We’re already very confident that there was no malicious intent or cyberattack involved,” he added.
More than 900,000 customers have had their power restored following the blackouts, which affected many more people at transport hubs and on the roads.
Ofgem said "enforcement action”, which can include fines and other penalties, could be imposed as a result.
Around 300,000 UK Power Networks customers were affected in London and the South East 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 Power grid, 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.