The six largest companies, which account for 90% of the market, all ranked towards the bottom of the table in the latest Which? energy company satisfaction survey.
Npower achieved the lowest score for the fourth year running with 35%, rating the worst for its complaint handling and customer service.
Scottish Power achieved a slightly higher score (41%), below EDF Energy (49%), British Gas (49%), E.ON (50%) and SSE (50%).
Scottish Power and Npower both fell well below the British industry average of 48%, while Spark Energy was the only smaller supplier to score as poorly as the Big Six with 50%.
Consumers ranked Ecotricity the best supplier with a score of 84%, closely followed by Good Energy (82%), Ebico (81%), Ovo Energy (80%), Utility Warehouse (76%) and Flow Energy (73%) – all smaller independent providers.
Which? rated companies on criteria including value for money, customer service, complaints handling, clarity and accuracy of bills and how they help their customers to save energy.
It based customer scores on overall satisfaction and the likelihood of recommending the company to a friend.
The last three Which? annual surveys have all seen smaller suppliers rank higher than their Big Six counterparts.
The watchdog noted that the overall customer score for energy companies improved from 41% last year to 48%, but said this was still low compared to some other products and sectors it investigates.
The survey found suppliers were still failing on the basics of customer service such as dealing with complaints or ensuring bills were accurate and clear.
Which? executive director Richard Lloyd said: “For the fourth year running, smaller suppliers are wiping the floor with the Big Six on customer service.
“The large energy firms, which dominate the market, need to up their game as millions of customers deserve better.
“We need the Competition and Markets Authority to propose radical remedies to fix this broken market.
“Instead of waiting for the outcome of the competition inquiry, companies should make immediate improvements to help restore trust among their long-suffering customers.”
The survey follows Scottish Power, British Gas and E.ON cutting household gas tariffs in the face of pressure to pass on the falling cost of energy on wholesale markets.
Energy firms have come under pressure from politicians as well as regulator Ofgem to pass on the benefit of sharply lower wholesale prices.
A Department of Energy and Climate Change spokeswoman said: ” Our reforms are working and the market is getting more competitive. The number of independent suppliers has almost trebled since 2010 and there’s now more pressure on energy companies to up their game and make sure they’re putting their customers first.”If people aren’t satisfied, now is the time to act – shop around for a better deal, switch suppliers and save.”
Gary Smith, national secretary for energy at the GMB union, said: “GMB members in customer services are not surprised at these figures.
“GMB members report that waiting times, complaints and customer frustration will continue to increase if the Big Six don’t address the chronic under-staffing in customer service centres.
“The Big Six continue to pay lip service to UK consumers, increasingly undermining service standards by cutting frontline jobs through offshoring and outsourcing.
“Instead of investing in UK workers, the Big Six are increasingly forcing customers to use digital alternatives such as ‘live chats’, without telling consumers they are dealing with offshore workers.
“Whenever new technologies are introduced the Big Six immediately move to reduce staff numbers in order to maintain profits, rather than reduce workload which would result in lower call waiting and handling times for consumers.
“GMB is also calling on the CMA investigation to take proper account of service standards in order to protect UK consumers, workers and their communities.”
Shadow energy secretary Caroline Flint said: ” These figures are a damning indictment of the Big Six energy companies and a broken energy market that is failing to meet the needs of consumers. Under David Cameron, millions of consumers are on the receiving end of poor customer service and rip-off energy bills.
“The next Labour government is committed to making big changes in our energy market, freezing energy prices until 2017 so that bills can fall but not rise, and creating a tough new regulator with the power to force energy companies to cut their prices when wholesale costs fall and to strip energy companies of their licences if they repeatedly mistreat customers.”
Industry body Energy UK said: “C ustomer service is a key priority for energy companies, which is why they have been investing in new systems, staff and new cheaper tariffs. Change takes time in an industry as large as the energy sector.
“It is heartening to see that Which?’s customer satisfaction score has improved; however, we do not under-estimate the challenges ahead. This comes at a time when the best tariffs are up to 10% cheaper than this time last year.
“No-one wants to see complaints and most are dealt with by the end of the next working day with no more than a phone call. The majority of queries relate to billing issues, a problem that will be significantly reduced with the roll-out of smart meters.”