Soaring prices and poor customer service have contributed to a surge in complaints from customers - with npower receiving a higher rate of customer dissatisfaction than any other "big six" company.
Latest research compiled by Consumer Futures, which represents consumers in regulated markets, said npower had 202.5 complaints per 100,000, compared with 38.3 per 100,000 for SSE, the lowest of the main energy providers, from the period April to June 2013.
The figures do not include any complaints made in the wake of recent price hike announcements.
Audrey Gallacher, director of energy at Consumer Focus, said: "Complaints about npower over recent months put it in bottom place. The company is implementing system changes that inevitably caused disruption to customers, however its complaints performance is unacceptable and the company must take further steps to tackle this.
"Energy companies have repeatedly said they want to rebuild consumer trust. Along with price, good service is important to customers. People want to know the relative performance on complaint handling to help them make informed choices when deciding whether to switch.
Public confidence in energy suppliers has been dealt a major blow since the start of last month when five of the Big Six energy firms announced that charges would rise by an average of around 9%. Suppliers have blamed wholesale energy costs, network charges, and government green levies.
ScottishPower said it will raise gas tariffs by 8.5% and electricity prices by 9% on average from December 6, adding another £113 to the typical annual dual fuel bill.
British Gas, which operates in Scotland as Scottish Gas, said government energy schemes are one reason it is hiking prices by 10.4% for electricity and 8.4% gas from this month, while EDF Energy said last week that average prices would rise by 3.9% in January.
SSE said more than seven million customers will be hit with an 8.2% rise - which came into force from last week - while RWE npower will put up electricity and gas prices by 9.3% and 11.1% respectively from December 1, affecting about 3.1 million customers.
It came as RWE said last this month it will cut 6,750 jobs across Europe and reduce costs by £840 million under fresh efficiency measures. There has been speculation that npower is to announce job cuts in the UK and will outsource work abroad.
Citizens Advice chief executive Gillian Guy said: "Price hikes of 36% over the last three years, coupled with poor customer service, has compounded the lack of trust in energy firms as households struggle to afford to have a warm home.
"The Citizens Advice service deals with 90,000 energy problems each year. People are being hit with large back bills, find their bill unclear and direct debit customers are frustrated companies are holding onto their money when they're in credit. Energy companies need to put customers first and be much more transparent about where they make their profits."
The study ranked SSE as having the lowest rate of complaints, ahead of Scottish Power (41 complaints per 100,000 customers), British Gas (55.5), E.ON (59.9), EDF Energy (75.5) and npower.