Ofgem has launched an official investigation into the power cuts that caused travel chaos across large parts of the country and left thousands of homes in darkness a fortnight ago.

It said the investigation will seek to establish what lessons can be drawn from the outage to ensure that steps can be taken to further improve the resilience of Britain’s energy network.

On August 9, nearly a million people across the UK lost connectivity after two National Grid generators spectacularly failed, with large parts of London, the South East, Liverpool, Glasgow, Wales, Gloucestershire and Manchester all left without power.

The regulator said as a result, it will also seek to establish whether any of the parties involved – the National Grid ESO, National Grid Electricity Transmission, 12 distribution network operators in England and Wales and generators RWE Generation (Little Barford Power station) and Orsted (Hornsea) - breached their licence conditions.

It will review the current requirements around 'back-up power', as well as how generators met their obligations on 9 August itself.

Under the Electricity Act, this could be a penalty of up to of 10% of the supplier's UK turnover
 

Generators have rules on how they should respond to these faults - and the investigation will look at whether these were met.

An Ofgem spokesman added that the watchdog will be looking at whether the companies were right to disconnect thousands of people - including the damage caused to rail passengers.

As the cuts spread through stations, commuters described "apocalyptic" scenes - with even traffic lights knocked out in some areas.

Trains were stuck on Thameslink and the East Coast main line, with delays and cancellations continuing all evening.

Meanwhile at King's Cross, 1,000 people were shut outside and others slept on the floor inside as train companies told ticket holders: "Do not attempt to travel."

Euston Station was also evacuated at the height of rush-hour along with passengers travelling at Newcastle Airport.

Jonathan Brearley, at Ofgem, 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.

“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.”

It said it will take enforcement action where suppliers failed to meet their obligations.   

Under the Electricity Act, this could be a penalty of up to of 10% of the supplier's UK turnover.

The August power cut was the first major outage since 500,000 homes lost power in London in 2003.