National Grid has blamed a lightning strike for Britain’s biggest blackout in more than a decade after it caused two power generators to go offline.

The strike was one of many to hit the grid on the same day as the 9 August blackouts but National Grid claims the bolt hit a transmission circuit north of London and managed to bring down two electricity generators more than 100 miles apart in an “extremely rare and unexpected event”.

A report revealed National Grid was unable to cover the twin outages at a gas-fired power plant in Bedfordshire and an offshore wind farm off the east coast of England because it did not have enough backup.

The outages took place within seconds of each other immediately after the strike, according to the report, causing disruptions across much of England and Wales.

The report said the Hornsea offshore windfarm shut down after experiencing an unusual voltage fluctuation “coincident with the lightning”. A spokeswoman for Ørsted, the windfarm owner, declined to comment.

RWE, the owner of the gas plant, has warned that it is too soon to say whether lightning had any impact on its operations because investigations into the root cause of its outage are ongoing.

Prof Dieter Helm, a government adviser on energy policy, said: “The key point is that the power cut should never have happened in the first place. If power cuts can happen when just two power generators drop off, then something fundamental has gone wrong.”

National Grid had only 1,000MW in reserve as backup supply when the outages took place. The combined power capacity lost in the twin outages was more than 1,300MW.

In addition, the strike may have also triggered outages at a string of “embedded” generators – such as small-scale renewables and diesel farms – totalling an estimated 500MW.

These off-grid generators are not visible to National Grid, which can only estimate the amount of electricity they add by noting a fall in demand for on-grid electricity when they run and a rise in demand when they shut off.

The energy regulator has responded by launching an investigation into whether National Grid’s statutory requirements, which are set by Ofgem, require it to hold enough reserve electricity supplies to stabilise the grid following outages.

The Guardian revealed National Grid had suffered three blackout “near misses” in as many months after similar-sized outages in May, June and July, which were all greater than 1,000MW.

Ofgem will also investigate whether the energy generators and regional energy networks responded correctly to the sudden loss in frequency caused by the power plants’ trips.

Jonathan Brearley, a senior executive at Ofgem, said the regulator had received the report and believed there were still areas that needed to be investigated.

“This will ensure the industry learns the relevant lessons and clearly establish whether any firm breached their obligations to deliver secure power supplies to consumers,” he said.

“The power cuts of Friday 9 August 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.”

A separate investigation is being conducted by Govia Thameslink Railway (GTR) and the manufacturer Siemens to discover why its new trains were so badly affected by the outage. The Class 700 and 717 trains, which have been introduced on Thameslink and Great Northern services over the last three years, shut down following the drop in frequency, although Network Rail’s overhead power line supply operated continually, according to the National Grid report.

While around half of the 60 affected GTR trains were restarted by their drivers, the rest required engineers to be dispatched, blocking tracks and causing huge disruption on lines into London St Pancras and King’s Cross. In total, 371 services were cancelled and 873 delayed, although other train types were unaffected by the power issues.

Steve White, chief operating officer of GTR, said that “a drop in the frequency of the overhead line voltage during the National Grid power failure… triggered a safety mechanism on all operational Class 700 and Class 717 Siemens Desiro City trains which caused the trains to shut down, protecting the onboard systems and electronics.”

He said Siemens and GTR had set up a task force to analyse how their trains reacted, and to minimise the impact of any similar incidents in the future.