The energy company, which last month launched a major marketing campaign to promote action on climate change, was rapped by industry regulator Ofgem.
It found that since 2006, EDF had failed to meet the three-month connection deadline stipulated by the regulator in over 100 cases.
Ofgem managing director Sarah Harrison, says: “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 service Ofgem will take tough regulatory action.”
However, the regulator says EDF has since taken steps to improve its connection service, “but they should have taken this action some time ago”.
The energy company last month launched a campaign to promote “Green Britain Day” – its latest attempt to encourage action on climate change.
Green Britain Day took place on 10 July aimed at getting more people involved in environmentally-friendly practices.
But rival energy suppliers have hit out at the initiative, launching legal action over the Green Britain Day green-coloured union jack logo.
EDF is the official sustainability partner of London 2012 Olympic Games.