Following a 74 per cent rise in the amount of complaints lodged against EDF, Consumer Focus has awarded the supplier with a zero star rating - marking the first time no stars have been awarded.
Consumer Focus - which is backing the upcoming consumer education-focussed Big Energy Week - also found that for every 100,00 customers that EDF had, almost 220 complained between July and September - a figure more than twice that received by nPower, who were ranked as the next-worst energy supplier.
Both E.ON and nPower also saw their ratings fall; meanwhile Scottish Power were the only one of the "big six" energy suppliers to display an improvement in the satisfaction amongst their customers.
EDF attributed their poor performance to a change in billing systems and were quick to add that there had been an improvement since October.
A spokesman for the firm said: "We sincerely apologise to those customers who have experienced any problems during this temporary period and thank them for their patience.
"As we finish the transition in the next month or so we will strive to maintain the improved service while we finish the job."
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