BRITISH Gas has seen a surge in complaints since it introduced a new billing system, an energy watchdog said.
Between October 2006 and March 2007 the company had 21,427 customer complaints - a 167% rise on the amount for the same period last year, when there were 8,012.
Last month alone, more than 14,000 people complained.
Energywatch said the majority were from customers disputing their bill, including problems with incorrect meter readings and out-of-date account details.
Problems have occurred because customers are receiving estimated bills for greater amounts of gas or electric than they are actually using.
Instead of having their meter read, customers are sent a bill, calculated by a computer which has been programmed with a formula to estimate how much they have used.
Energywatch campaigns manager Adam Scorer said: "When customers have tried to complain about the inaccuracies in their bills they haven't been able to get through to a British Gas person on the phone.
"Sometimes it takes them an hour and they have to come back time and time again before they get any sort of outcome.
"The inability to treat their customers who have a genuine problem with their bills with the most basic level of customer service is what's causing British Gas's reputation, quite frankly, to be dragged through the mud."
British Gas admitted today that its service had not been as good as it should be for some customers.
A spokeswoman said: "Our new managing director Phil Bentley, who started work in March, has made a personal commitment to putting our customer service level back to where it should be as one of the UK's top rated energy suppliers and we're already seeing clear signs of improvement, having recruited an additional 800 staff to help call answering."