A lightning strike at Eaton Socon is believed to have triggered a meltdown which left more than a million electricity customers without power, a report has said.
Little Barford power station operator RWE Generation is being investigated by the regulator Ofgem over its role in the power failure which led to rush hour trains from King's Cross being suspended, hitting passengers from Huntingdonshire, with a hospital and an airport also being affected.
Now Ofgem has launched an investigation into the "extremely rare and unexpected" power cut on August 9 so that steps can be taken to improve the resilience of Britain's energy network.
It will also seek to establish whether any of the supply organisations, including RWE Generation, breached their licence conditions.
RWE Generation said: "Investigation into the root cause of the incident is ongoing. At this stage we believe the turbine generator automatic control system detected an abnormality and initiated a safety shutdown of the steam turbine, followed by the loss of two gas turbines. We await the final reports into the power cuts from all parties."
A preliminary report by the National Grid Electricity System Operator (ESO) said there had been heavy rain and lightning on August 9.
At 4.52pm lightning struck the transmission circuit at Eaton Socon, causing protection systems to come into effect, briefly cutting supply before the line returned to normal.
But immediately after the strike and within seconds of each other, Little Barford power station and the Hornsea off-shore wind farm reduced their energy supply to the grid - setting off a reaction which led to 1.1 million customers losing their electricity supply.
The report said reserves of power held to cover problems were used up because of the scale of the generation loss, setting off a controlled disconnection scheme.
"This has not happened in over a decade and is an extremely rare event. This resulted in approximately 1.1 million customers being without power for a period," the report said.
It said a type of train operating in the region "reacted unexpectedly" to the cut, with half the 60 in use requiring an engineer to go out and restart them, leaving passengers marooned. Ipswich hospital and Newcastle airport also lost power.
Jonathan Brearley, Ofgem's executive director of systems and networks said: "The power cuts caused interruptions to consumers' energy and significant disruption to commuters. It's important that the industry takes all possible steps to prevent this happening again."
Ofgem's investigation follows the submission of an initial report into the power failure by the ESO and further action could be taken by the regulator after the National Grid publishes its technical report by September 6.
It could impose penalties of up to 10 per cent of UK turnover on regulated companies deemed to be at fault.
The regulator is also working with its rail counterpart to find out why the drop in power on the energy network led to disruption for passengers.
Ofgem's investigations will initially focus on the requirements to hold sufficient back-up power to manage loss of supplies and how generator and distribution organisations met their obligations during the cut.
The ESO's preliminary findings said the almost simultaneous and unexpected power losses at Little Barford and Hornsea occurred independently from each other but were associated with the lightning strike: "As generation would not be expected to trip off or de-load in response to a lightning strike, this appears to represent an extremely rare and unexpected event."
It said there were numerous lightning strikes during the day, but this was the only one to have a significant impact and that protection systems operated correctly.
However, there was an "exceptional cumulative level of power loss" outside the normal range which network operators dealt with, restoring supplies within 31 minutes once the system had stabilised. It said some services, including rail, were affected for longer by the action of their own systems.
The ESO said its final report to Ofgem would include the exact failure mechanisms at Little Barford and Hornsea, its work with network operators on the loss of supply and a review of its communications.