Energy companies have been fined after leaving Hertfordshire commuters stranded in August when the train network failed.

The energy regulator Ofgem ruled today that RWE Generation, Orstead and UK Power Networks will have to pay £10.5 million, after the combined loss of two large generators triggered disruption to more than one million consumers.

Jonathan Brearley, executive director at Ofgem, said: "Consumers and businesses rely on generators and network companies to provide a secure and stable power supply.

"August 9 showed how much disruption and distress is caused to consumers across the UK when this does not happen.

"That is why it is right that companies that were unable to keep generating have paid into our consumer redress fund.

"Our investigation has raised important questions about National Grid's Electricity System Operator, which is why our review will look at the structure and governance of the company.

"As the energy market changes it is vitally important we futureproof the networks to ensure consumers continue to benefit from one of the most reliable electricity systems in the world."

The Office of Rail and Road, working with Ofgem, also found fault with Govia-operated Thameslink trains, which brought rail travel north of London to a standstill.

In a report released today, it said the permanent lock-out resulted in 30 Thameslink trains with upgraded software stranded on the rail network and in need of a technician to individually reset them.

It added that this caused "significant delays for the entire network" and "significant knock on effects" but the rail regulator concluded it did not warrant enforcement action.

The Office of Rail and Road also recommended that Siemens checks its software on the trains to prevent another lockout.

Steve White, Chief Operating Officer of Govia Thameslink Railway, said: "Passengers are now protected from this rare but disruptive event because, following a full review with GTR, Siemens Mobility has modified all the trains so that none of them now need a technician to restart."

Govia did pay the cost of travel by taxi back to Hertfordshire for many commuters and refunded the cost of their tickets.

The rail regulator said: "As a result, compensation payments claimed by the public increased but complaints did not."

Govia Thameslink has also admitted that the "widescale failure of the power grid" prevented their trains moving and their technicians were unable to solve the problem quickly.