The blaze damaged two transformers. Picture by Leighton Grant
Investigators have been working through the night to determine the cause of a fire that left thousands of homes in south London without power.
EDF Energy said "tens of thousands" of properties were affected when a fire started at an electricity sub-station in Sydenham on Sunday morning.
The blaze was under control by midday and power fully restored by 2200 BST.
An EDF spokesman said experts were working with the fire service to find out how the blaze started.
Up to 30 firefighters tackled the blaze which started at the sub-station in Sydenham Park Road, Forest Hill, at about 0720 BST.
Channels off air
Two of four transformers, located outside the building, were damaged by the flames.
As a result homes in Lewisham, Eltham, Sydenham, Brockley, Forest Hill, Catford, Bromley, Beckenham, New Addington and Orpington were left without power for most of the day.
Southern train rail services were also suspended and scores of traffic lights failed to work.
BBC1, BBC2 and BBC London 94.9 also went off air in some parts of London at about that time.
According to EDF, power was restored to the majority of properties by midday with about 200 homes being without power until around 2200 BST.
A spokesman said: "We were able to get one transformer back in use by about midday but we had to wait until the temperature had dropped sufficiently to restore power to the remaining homes."
London Fire Brigade and EDF have been examining the scene to establish the cause of the fire.