Npower has been rated the worst power supplier in Britain for the second year in a row.
In Which?’s annual energy satisfaction survey, the German-owned giant was given an overall score of 39 out of 100.
Npower, which has three million customers but has been continually criticised for poor customer service in surveys over recent years, scored two out of five stars in every category.
Ten thousand customers were asked to rate their supplier for value for money, service, complaints handling, bill accuracy, and energy saving advice.
Other members of the Big Six firms were given lowly ratings too. Only two of the six - which supply 99 per cent of households – scraped into the top 10, with E.ON and SSE coming joint ninth with an overall score of 51 out of 100.
British Gas, the country’s biggest supplier, received a score of 50, along with Scottish Power.
EDF-Energy’s 46 placed it second bottom, seven ahead of Npower, owned by the Essex-based conglomerate RWE.
The big suppliers were outclassed by the industry’s minnows in the survey. Good Energy and Ecotricity came top, scoring 85 and 80 out of 100 respectively. Other high performers were Ebico (76), Utility Warehouse (76), Ovo Energy (74) and Co-operative Energy (69).
Which?’s executive director, Richard Lloyd, said: “Yet again customers have told us they are fed up with the way the biggest energy companies treat them. The Government’s energy reforms need to go much further to increase competition so that suppliers are under pressure to raise their game.”
Yet he added that the complexity of tariffs made it hard for customers to shop around for a better deal.
Under plans by the Energy Secretary Ed Davey in November, suppliers will be limited to offering four core tariffs each instead of the hundreds currently on the market.
The Government believes simplifying the system will intensify competition and thus drive down prices and drive up standards.
Mr Lloyd said: “The Government's energy reforms need to go much further to increase competition so that suppliers are under pressure to raise their game and keep their prices in check.
“We want the switching process to be quicker and for energy prices to be set out in a way that allows people to spot the cheapest deal at a glance.”
Which?’s research shows that on average customers who switch suppliers save an average of £217 on their gas and electricity.
Npower chief executive Paul Massara said: “Clearly, I'm disappointed by the results and I'm sorry if some of our customers feel we have let them down in any way. My primary focus as new chief executive is to put the customer at the very heart of our business and improve customer satisfaction, and so we have started a comprehensive programme and cultural change programme to make sure that we deliver on our promises to customers”