Regulator Ofgem have fined EDF Energy £2 million for "poor services".
EDF were fined for failing to connect customers quickly enough to its electricity networks.
Ofgem found in over 100 cases the energy provider failed to make a connection offer in three months and had no system to determine if it was meeting targets.
The energy company, which has operations focused in London, the east and the south-east, and is the monopoly provider of certain connection services in these areas was told it was expected to offer good customer service.
Sarah Harrison, Ofgem managing director of corporate affairs, said: "Customers should not have to accept poor service in any part of the energy market.
"All energy companies should be in no doubt that if they are failing to offer good customer service Ofgem will take tough regulatory action."
She added EDF had now taken steps to improve its connections service, "but they should have taken this action some time ago".
EDF Energy has already paid around £450,000 in compensation to affected customers.
EDF Energy was in the headlines this week after 94,000 customers in south-east London were left without electricity for a number of days after a fire thought to be caused by vandals hit a cable bridge.
To provide energy, the firm rolled out the biggest deployment of mobile generators in the history of the London network and a rota system for electricity was introduced.
Customers are still being asked to use power for essential purposes only.
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