A lightning strike and the sudden loss of two large electricity generators were "partly to blame" for the blackout on 9 August, according to an interim report.
Ofgem said it wants to “establish what lessons can be drawn from the power cut to ensure that steps can be taken to further improve the resilience of Britain's energy network.”
It added that it will seek to establish whether 12 distribution network operators — as well as National Grid ESO, National Grid Electricity Transmission — in England and Wales breached their licence conditions.
If the investigation leads to a fine, Ofgem can impose penalties up to a maximum of 10% of the company in question’s turnover, under the Electricity Act. Firms can also be ordered to pay money to charities to help less well-off customers.
"The power cuts on 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,” said Jonathan Brearley, Ofgem's executive director of Systems and Networks.
"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.
“The opening of this investigation does not imply that Ofgem has made any findings about non-compliance for any of the companies mentioned at this early stage of the investigation. However, if evidence does emerge in the course of the investigation enforcement action could follow.”
About 300,000 customers in south-east England were left without electricity while a further half a million people in Wales, south-west England and the Midlands were also affected by the outage.
Some 110,000 in Yorkshire and north-east England, and about 26,000 in north-west England were also impacted.