Despite controlling just 30 per cent of the market, British Gas was the subject of 70 per cent of customer complaints to Energywatch between October 2006 and March 2007, compared to just nine per cent for Npower, six per cent for Scottish Power and five per cent for Powergen.

Most complaints centred on inaccurate billing caused by the switchover to a new billing system, it said.

Since the introduction of the new system, customers calling British Gas to complain have allegedly experienced poor customer service.

“Customers should not be left hanging on the phone or having to explain their circumstances to several different advisers,” protested Energywatch’s campaigns director, Adam Scorer.

What is more, the failure to send out accurate bills undercuts the positive impact of cuts to the basic tariff introduced last month, claimed SimplySwitch.com’s founder, Karen Darby.

But British Gas’ managing director Phil Bentley told BBC Radio that “99.7 percent of our customers are absolutely happy with the service,” promising “significant improvements in service by summer” for disgruntled customers, Reuters reports.

Find out more about how you can compare gas and electricity prices

© Adfero Ltd