Complaints at five of the UK’s largest energy firms rose by an average of 26% over the autumn. Out of the Big Six firms – SSE, British Gas, npower, E.ON, ScottishPower and EDF – all but Scottish Power saw an increase, according to Consumer Focus.

French supplier EDF Energy came out worst in the consumer body’s latest complaints league table, with complaints up 74% in three months and 91% compared with this time last year.

Consumer Focus suggested this was likely to be down to teething problems with EDF’s new billing system.

Adam Scorer, Director of External Affairs at the consumer body said: “Complaints about EDF Energy over the summer have had a catastrophic impact on its rating. While system changes inevitably cause disruption to customers, this must be minimised. Its current complaints performance is unacceptable and the company must take further steps to tackle this.”

The figures from July to September 2011 suggest consumers are struggling to see eye to eye with power firms at a time when most have introduced unprecedented price rises.

Mr Scorer added: “It is disappointing, but perhaps not surprising, that complaints on energy issues have risen at a time when energy bills are increasing. Energy companies have repeatedly said they want to rebuild consumer trust. Good customer service and complaints handling are key ingredients to building consumer trust but suppliers still have a long way to go.”

EDF expressed “disappointment” in the result and apologised to customers.

It said in a statement: “We are obviously disappointed that we have not been in a position to consistently deliver the high levels of service this year that we expect of ourselves. We sincerely apologise to those customers who have experienced any problems during this temporary period and thank them for their patience.”