Customers of the Big Six energy firms could have saved up to £234 a year by switching suppliers, the Competition and Markets Authority (CMA) has said.
The watchdog is currently investigating the UK's energy market to see if it's providing a fair deal for consumers amid concerns of the dominance of the major suppliers.
In an update on its findings so far, the CMA said more than 95% of dual fuel customers of the Big Six would have saved between £158 and £234 a year if they switched provider between 2011 and 2014.
Long-term customers on standard variable tariffs tend to be charged the most for their energy as they don't consider moving to a different supplier, it said. Between 40% and 50% of customers have been with a supplier for 10 years or more, giving an indication of how little people switch suppliers to get better rates.
"Customers on standard variable tariffs are less likely to have considered switching, shopped around, switched provider or switched tariff with an existing provider, less educated, less well-off and more likely to describe themselves as struggling financially," the CMA said.
The report adds that there is "considerable" concern over the quality of service customers receive from the Big Six, with the number of recorded complaints increasing five-fold between 2007 and 2013. Most complaints related to billing, customer services and back billing.
"We note that increasing numbers of complaints may reflect: declining quality of service; price rises; changes in reporting standards; increasing media scrutiny of the sector; or a combination of these factors," it said.
The Big Six - SSE, Scottish Power, RWE, Centrica, EDF, and Npower - account for around 92% of the UK energy market. The CMA will publish its initial findings and conclusions in May.