Npower has been criticised for being the most complained about energy firm in the country.
According to new research, the energy supplier was the worst ranked out of all the 'Big Six' firms - with customers complaining about a number of issues including poor customer service.
The research by Consumer Futures, claimed npower had an incredible 202.5 complaints per 100,000 customers, compared to the lowest figures of 'Big Six' firms, SSE, which had an average figure of 38.3 complaints per 100,000.
Consumer Focus director of energy, Audrey Gallacher, 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."
Between the lowest level of complaints from SSE and the highest level of complaints from npower were Scottish Power, which had 41 complaints per 100,000 customers, British Gas with 55.5 per 100,000, E.On with 59.9 per 100,000 and EDF Energy with 75.5 per 100,000.
The figures for the research were taken between April and June 2013 and with five out of six of the 'Big Six' bumping up prices in the last month, public confidence with the energy suppliers is only likely to diminish further.
Audrey Gallacher added: "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.
"Customer satisfaction with how complaints are handled is low across a whole range of industries and the same problems are seen over and over again. Companies must remember that many people view how a firm handles their complaint as a sign of how valued they are as a customer - so it is essential to deliver on this key issue."
However a spokesman for Energy UK, the representative of the UK's energy firms, defended the 'Big Six' and claimed most customers were more than satisfied with their energy provider.
He said: "The vast majority of energy customers are happy with the service they get with only around one in every 1,400 customers likely to need to contact their supplier about a problem.
"Most complaints only need a phone call to sort out - around four out of five queries are resolved by the end of the next working day - but, if the problem cannot be resolved, the energy ombudsman is there to ensure problems get fixed.
Energy companies take their relationship with customers extremely seriously and work hard to improve customer service."
Latest gas and electricity news brought to you by UK Power - the energy price comparison site.
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.