A lightning strike on a transmission circuit followed by “almost simultaneous unexpected power losses” at two plants caused the blackouts on August 9, an interim report has found.

The outages at Hornsea off-shore windfarm and Little Barford gas power station occurred independently and the scale of generation loss meant that the level of “backup” power required under regulations was insufficient to cover the loss, the report from National Grid Electricity System Operator said.

As a result, the system automatically disconnected customers on the distribution network with about 5% of electricity demand being turned off to protect the other 95%.

It meant major problems for Merseyrail with all trains on Wirral lines coming to a standstill. Traffic signals across Walalsey were also blacked out.

The interim report said: “Two almost simultaneous unexpected power losses at Hornsea and Little Barford occurred independently of one another – but each (was) associated with the lightning strike.

The report said the lightning strike was one of many that hit the grid on the day and strikes are “routinely managed as part of normal system operations”.

It said protection systems “operated correctly to clear the lightning strike and the associated voltage disturbance was in line with what was expected”.

Responding to the report, industry regulator Ofgem said its investigation into the power cuts would examine the grid’s requirements to hold sufficient back-up power to manage loss of generation supplies, how generators met their obligations with respect to the transmission fault and whether distribution network operators complied with their obligations.

Ofgem also said it would be “looking at whether the companies made the right decisions both in the numbers of customers disconnected and whether those customers disconnected were the right ones”.

Ofgem said it was liaising with the rail regulator and rail authorities “to understand better why the drop in frequency on the energy network led to disruption for passengers”.

Jonathan Brearley, Ofgem’s executive director of systems and networks, said: “It’s important that the industry takes all possible steps to prevent this happening again.

“Having now received National Grid ESO’s interim report, we believe there are still areas where we need to use our statutory powers to investigate these outages.

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