The majority of people are missing out on big savings by not switching energy supplier.
The interim findings from the Competition and Markets Authority (CMA) found millions of customers of dual-fuel could have saved up to £234 every year by switching tariffs.
The CMA is investigating the energy suppliers after a referral by the energy regulator Ofgem which is concerned about the dominance of the six companies in the energy market. They account for 92 per cent of the market.
Poor customer service was also an issue for the big six, with the number of complaints rising from fivefold between 2007 and 2013 which could reflect any number of factors the CMA said, including the declining quality of service, price rises, changes in reporting standards or increased media scrutiny on the sector, or a combination of them all.
Ofgem welcomed the CMA update and said it would consider the findings before responding.
The inquiry, which is due to conclude later this year, will next look into the profits at energy firms and the decision by Ofgem to reduce the number of tariffs available to consumers.
Centrica and SSE shares edged up slightly on the news as the industry awaits the full findings.