Ofgem, the energy regulator, has widened its investigation into EDF's customer care practices by putting three specific call centres under the spotlight.
One in eight of the company's 5.5 million customers lodged complaints to the energy ombudsman in the 12 months to October 2011, the highest level registered against one of the leading six energy providers.
An Ofgem spokesman said: "Ofgem has been investigating whether EDF Energy has breached regulations setting out how customer complaints must be dealt with. Shortly before Christmas 2011 we widened the investigation to consider whether EDF Energy's customer contact centre is complying with the requirements of the regulations."
EDF had suffered problems last year with a new IT system which had led to an increase in call waiting times. The company took on an extra 700 customer services staff to help through this transition period, but the company claims its call waiting times spiked "as a result of higher than anticipated call volumes, in part because of the industry-leading prices we had on offer."
Last year, EDF rivals British Gas and npower were both slapped with £2.5m and £2m fines respectively by Ofgem for poor complaints handling. Ofgem imposed new rules on the industry in 2008 governing the mishandling of complaints to set minimum standards on how customers should be treated.
The common breaches of the regulator's regulations have been the failure of energy firms to explain fully and transparently the role of the energy ombudsman and completing the complaints procedure properly to resolve the issues raised by the customer.