Customer complaints about the Big Six energy companies have reached a record high.
Some 1.7 million consumers complained about one of the Big Six in the first three months of 2014 - the highest level of complaints since complaints began being recorded in 2012 and up from around 1.5 million on the same period last year.
According to Which?, the most complained about firm was Npower. It received 83 complaints for every 1,000 customers, an increase on 49 complaints per 1,000 on the same period for 2013.
Scottish Power had the least recorded complaints, with just 13 per 1,000 customers, while SSE, British Gas and E.ON all had around 30 complaints per 1,000.
However, British Gas and SSE's number of complaints had doubled over the last six months, while EDF's dropped from 77 per 1,000 customers this time last year to 46.
Which? executive director Richard Lloyd said the figures are yet another example of a failing energy market and urged them to do more to improve the service consumers receive.
"Yet again millions of customers are being let down by poor service from the big six energy companies," he said.
"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."
The Big Six firms supply around 95% of the UK's energy and watchdog Ofgem is currently investigating whether anti-competitive practices in the energy market is leading to consumers getting a raw deal.
This week it was announced consumers unhappy with their current provider will be able to switch to another company in just three days by the end of 2014.
In response to the findings, a spokesperson for Energy UK, the industry's trade body, said: "Most customers are happy with their energy provider. But, in an industry which serves 27 million households, sometimes things can go wrong."