An inquiry is underway into a huge UK power outage partly blamed on system disruption at the Hornsea 1 offshore wind project – the world’s largest – that is currently under construction off Eastern England.

Lost generation from Hornsea 1, which will total 1.2GW when fully commissioned, and a gas plant were pinpointed as the twin causes of a blackout that hit a huge swathe of Britain late on Friday afternoon, sparking travel chaos at peak commuter time.

Orsted, the Danish offshore wind giant building Hornsea 1, said: “Hornsea One lost load on Friday 9 August, around the same time as another generation source lost load. We are investigating the cause, working closely with National Grid System Operator, which balances the UK’s electricity system.”

UK regulator Ofgem ordered an “urgent, detailed report” from TSO National Grid into the cause of the outage, which lasted less than an hour but whose impact was being felt far longer.

National Grid said: “The root cause of [the] issue was not with our system but was a rare and unusual event, the almost simultaneous loss of two large generators, one gas and one offshore wind, at 16.54pm. We are still working with the generators to understand what caused the generation to be lost.

“Following the event, the other generators on the network responded to the loss by increasing their output as expected. However due to the scale of the generation losses this was not sufficient, and to protect the network and ensure restoration to normal operation could be completed as quickly as possible, a backup protection system was triggered which disconnects selected demand across GB.”

Hornsea 1 by early July had installed 100 of its 174 Siemens Gamesa 7MW turbines, and began exporting power to the grid in February.