London's Mayor Ken Livingstone has described a power cut which caused rush-hour misery for 250,000 people as an "absolute outrage".
Some Tube stations were evacuated
He warned the results could have been worse if the breakdown had struck during the recent hot weather - as the National Grid apologised for the failure.
Energy Minister Stephen Timms has asked for an urgent report on the blackout, which started at 1826 BST on Thursday, stopped about 1,800 trains and closed 60% of the Tube network.
The emergency was triggered when an alarm indicated a fault at the Hurst substation near Bexley in Kent at 1810 BST.
National Grid switched the transformer out of service at 1820 BST and power was still able to flow into London through other circuits.
But another fault within seven seconds stopped flows on a 275,000-volt underground cable between the New Cross and Wimbledon substations.
The National Grid says it may be some weeks before the exact nature of the fault is known.
Network Rail said services were unaffected by the power cut on Friday morning, despite disruption continuing late into Thursday night.
London Underground (LU) said the Tube was running mainly as normal, apart from some delays on the Piccadilly and District lines.
National Grid chief operating officer Mark Fairburn said: "We
would like to apologise for the inconvenience caused to people."
Engineers were still trying to discover the cause of the problems at substations in Wimbledon, New Cross both in south London and Hurst in Bexley, Kent.
Speaking on the BBC Radio 4 Today programme, Mr Fairburn said a full investigation was under way.
"We had an equipment failure on our system in south-east London and that was followed within seconds by a second fault which caused the power cut," he said.
But a furious Mr Livingstone said there could have been "horrifying consequences" if it had happened during the hot spell a few weeks ago.
He told the Today programme that structures had been put in place to ensure the power system could cope with a terrorist attack, but in this case it could not cope with normal demand.
He added that a year or so ago LU had had its own power supply, but this had been axed - "presumably as an economy measure" - and it now had to rely on the National Grid.
"Under-investment in the National Grid must not be allowed to cause this kind
of chaos in a city like London," he said.
But Mr Fairburn said: "This has got nothing to do with under-investment. Investment levels are at the highest level the industry has seen."
Professor Ian Fells, an energy adviser to the government and the World Energy Council, said: "It proves that the system is frail and it needs more investment."
He said electricity prices have been driven down so much that energy companies were not making profits to invest back into the system and build extra power supplies.
POWER CUT SOURCE
See how the substations hit by Thursday evening's fault fit into London's electricity grid
Power was fully restored some two-and-a-half hours after the
Most areas hit had power restored within 30 minutes.
Pubs lit candles as people left work to seek refuge, many staying late into the night as crammed buses and taxis tried to help thousands of people to get home.
The BBC's transport correspondent Tom Symonds said: "It showed the vulnerability of London."