National Grid Transco has said last month's power blackout in south London was partly caused by incorrectly installed equipment.
The company said a key piece of protection equipment at its Wimbledon substation did not have sufficient capacity to cope with a change in power flows caused by the routine disconnection of a faulty transformer.
The failure of the back-up system triggered a power blackout across a swathe of south London during the evening rush hour on 28 August, leaving thousands of commuters stranded on the capital's Tube network.
It was the largest loss of supply on National Grid's network for over 10 years.
In its official report into the incident, National Grid said the equipment had been incorrectly installed during an infrastructure upgrade in June 2001, despite "rigorous" checks.
"This incorrect installation was not discovered despite extensive quality control and commissioning procedures followed by both the supplier's and National Grid's specialist staff," the company said in a statement.
National Grid added that it was inspecting the rest of the protection equipment on its network to make sure that it did not have similar faults.
The inspection, currently 20% complete, has not so far revealed any irregularities, it said.
National Grid chief executive Roger Urwin said the investigation would not have a significant impact on the company's profits.
Separately, the government said it would carry out its own investigation into the London power cut, and a second blackout which hit the West Midlands on 5 September.
The probe, to be carried out jointly by the Department of Trade and Industry and energy market regulator Ofgem, will focus on ways of preventing similar power failures in future.
"We need to understand whether improved communication between National Grid, the distribution companies and their customers could have reduced the disruption caused," said Energy Minister Stephen Timms.
Ofgem is expected to publish an initial assessment of the London and Midlands power failures next month.