Labour have renewed their fight against the UK's big energy firms after a probe found that the suppliers' customers have missed out on big savings by not switching providers.

The Competition and Markets Authority (CMA) investigation revealed that more than 95% of the dual-fuel customers of the six large energy companies could have saved up to £234 ($359, €315) a year by switching tariff.

The "big six" energy firms – SSE, Scottish Power, Centrica, RWE Npower, E.On and EDF Energy –account for around 92% of the UK's energy market.

"This report confirms that Britain's energy market is broken and that radical action is needed to protect consumers," said Caroline Flint, the shadow energy and climate change.

"Energy bills are £300 a year higher under the Tories, and David Cameron has let the energy companies get away with overcharging millions of consumers."

Flint explained that a Labour government would "freeze" energy prices until 2017.

The latest findings from the CMA investigation, which was launched last year after a referral from Ofgem, also found that shed light on the barriers of switching tariffs.

The report said that people who were more likely not to change suppliers were those aged 65 and over those in social accommodation; those with no qualifications; and those on lower incomes.

"We recognise that, for some customers, failing to switch even when there are savings to be made may not be an indication of a competition concern," the CMA said.

"They may place sufficient weight on the quality of service provided by their current supplier, for example, for this to outweigh the financial gain on offer.

"Therefore, a major focus of the next stage of our investigation will be on understanding which sorts of customer do not switch and why, and in particular on identifying the nature of any barriers to switching they face."