An incorrect piece of equipment caused the power cut which plunged much of London and the South East into darkness, according to a preliminary report by the National Grid.
Some Tube stations were evacuated
The report says it was the largest power cut from the National Grid for 10 years and the company expressed "its deep regret".
About 1,600 trains were stopped and 60% of the London Underground network was closed, stranding 250,000 during the evening rush hour.
Electricity was restored in 41 minutes and the outage affected 410,000 homes and businesses, according to the report.
The company says a failed transformer at the Hurst substation near Bexley in Kent on 28 August, triggered the outage.
National Grid switched the transformer out of service at 1820 BST and power was re-routed through other circuits so it could still flow.
But within seven seconds another fault occurred which stopped the flow of electricity on a 275,000-volt underground cable between the New Cross and Wimbledon substations.
There are usually two circuits operating between the substations but one was not functioning because of maintenance work.
Queues grew for buses and taxis
According to the report, the second fault occurred because automatic protection equipment shut off the power thinking there was a fault with the supply, because of the action taken to compensate for the defective transformer.
The report adds that this occurred because "an incorrect protection relay was installed when old equipment was replaced in 2001".
National Grid has so far checked 9,000 out of 45,000 other similar pieces of equipment across the network and found them to be working as normal.
The remaining equipment will be checked within four weeks.
But it added: "The incident on the areas of south London was exacerbated by the loss of supply to the [London] underground and railway services."
It comes as the Department of Trade and Industry and energy regulator Ofgem said they will appoint specialist consultants to carry out initial fact-finding investigation into the blackout.
The inquiry will also cover blackouts in the West Midlands last Friday and power failures in the last five years to see if any trends can be identified.