Customer complaints to the Big Six energy companies have reached the highest number in a single quarter since records began - up 15% on the same time last year.
The big suppliers received a total of 1.7 million complaints in the first quarter alone, up from 1.48 million in the same period last year, consumer group Which? said.
Npower received 83 complaints for every 1,000 customers in the first quarter of this year - the highest number among the six companies and up from 49 complaints per 1,000 at the same time last year.
Scottish Power received the fewest complaints at 13 for every 1,000 customers. SSE, British Gas and E.On all received around 30 complaints per 1,000 customers, with SSE's figures doubling from 13 to 27 complaints per 1,000 on the same time last year. EDF's complaints dropped from 77 per 1,000 customers this time last year to 46.
The companies - British Gas, EDF Energy, E.On, Npower, Scottish Power and SSE - have published customer complaint figures since October 2012.
In February this year Which? reported that the Big Six received more than 5.5 million complaints between January 1 and December 31 last year.
Which? executive director Richard Lloyd said: "Yet again millions of customers are being let down by poor service from the Big Six energy companies. This has to change.
"If they want to improve the low level of consumer trust in the energy market, suppliers must up their game now, rather than wait for the results of a competition review."
An Energy UK spokesman said: "Most customers are happy with their energy provider. But, in an industry which serves 27 million households, sometimes things can go wrong.
"Most issues just take just a call to fix even though all problems - large and small - are lumped together. But all problems, no matter how minor, are important and if a customer has any concerns about their service, they should contact their supplier as soon as they can.
"No one wants to see complaints rise, but the industry uses this information to improve its service. Anyone with a problem needs to contact their supplier before they do anything else."
Shadow energy secretary Caroline Flint said: "These figures are a damning indictment of a broken energy market that is failing to meet the needs of consumers. Under David Cameron people are paying more than ever for their gas and electricity, but all too often the customer service they receive is not good enough.
"To help people with their energy bills, the next Labour Government will undertake the biggest overhaul of our energy market since privatisation. Our plans will break up the big energy companies, put an end to their secret deals and create a tough new regulator with the power to force companies to cut their prices when wholesale costs fall. And until these reforms kick in, we will put a stop to unfair price rises by freezing energy bills until 2017, saving the average household £120."
Energy and Climate Change Secretary Ed Davey said: "It is unacceptable that so many people have felt forced to complain to their energy supplier.
"Energy companies need to up their game - people are switching suppliers in unprecedented numbers, particularly to small suppliers whose numbers have nearly trebled since 2010."