The blackout in London was caused by two faults in quick succession on the National Grid.
Most of the Tube network ground to a halt without power
Firstly, at 1810 BST, a fault occurred at the Hurst substation in Kent.
The faulty transformer was switched off and power was able to flow into London using other circuits.
But within seconds another fault occurred, this time between the New Cross and Wimbledon substation, and it was too much for the system to handle.
Describing it as an "extremely rare event", the grid's chief operating officer said power had been restored within 40 minutes.
"We had an equipment failure in our system in south east London and that was followed in a matter of seconds by a second fault which caused the power cut," Mark Fairbairn told BBC Radio 4's Today programme.
"We have been undertaking a full technical investigation as to the background of the faults. That is still in progress."
Electrical engineer Dr Tim Green, of Imperial College, London, said it "looks like an unfortunate chain of events that will take some time to unravel".
SEQUENCE OF EVENTS
1810 BST: An alarm indicates a fault on a transformer at the Hurst substation in Kent
1820 BST: Transformer switched off but power still able to flow into London through other circuits
7 minutes later: Another fault occurs stopping flows on a 275,000 Volt underground cable between the New Cross and Wimbledon substation. Power black-out follows
1900 BST: Problem fixed and full power restored
"To be fair to the National Grid, they do put a lot of effort into planning their system and ensuring they have reserve capacity," he told BBC One's Breakfast programme.
"The UK has a pretty secure network on the whole."
But officials were facing uncomfortable questions on Friday about why there is no back up for the system.
Until a year ago, London Underground had an independent power station at Lots Road in west London.
It was switched off and the Tube network linked into the National Grid in a move which London Mayor Ken Livingstone blamed on cost-cutting.
He said there could have been "horrifying consequences" if the faults had occurred
during the very hot spell a few weeks ago, with many people hospitalised.
Mr Fairbairn denied a lack of investment was to blame.
Power supply to the Tube is the responsibility of Seeboard Powerlink, a private consortium.
It took over under the government's private finance initiative in 1998.
A spokesman for London Underground acknowledged that had a similar power cut in the National Grid happened when it still had an independent generator, Thursday's network breakdown would not have happened.
A small back up generator at Greenwich which is designed to power safety lights in trains and stations in the event of a total power failure, was not needed on Thursday because power was diverted from other parts of the system.
Meanwhile comparisons were being drawn with the catastrophic power failure which affected large swathes of north America and Canada two weeks ago.
Dr Green said: "There are two reasons why you might get a power cut.
"One is a mismatch between supply and demand and some of the talk in America was that that might have been at the bottom of the problem there.
"If you don't have enough supply in a system then you can go into a dramatic collapse.
"I think this looks more like an equipment failure."