Households across the UK are overpaying on their energy bills to the tune of £4 billion each year, according to new analysis.
Research has found that 12 million families are on standard variable tariffs with the Big Six energy suppliers, and overpay by an average of £350 a year.
Families in the South East and East of England are overpaying by the most, according to the analysis by energy supplier Bulb.
The Government is introducing an energy price cap to protect consumers on the most expensive energy tariffs.
The Domestic Gas and Electricity (Tariff Cap) Bill will put in place a requirement on Ofgem to cap standard variable and default energy tariffs until at least 2020 to tackle the amount consumers have been overpaying the Big Six energy suppliers.
The Bill enables the temporary price cap to remain in place until 2023 if conditions for effective market competition are not met.
However, millions of households are still facing higher energy bills this summer as providers raise prices.
E.ON, British Gas, SSE, Npower, EDF, ScottishPower and Bulb have all hiked energy prices, blaming wholesale energy costs for the increases.
Consumer watchdog Which? warned energy customers about the rising tariffs, saying in May that they should switch to a better deal or face overpaying by as much as £400 a year.
The dominance of the Big Six energy firms appears to be waning, however, as figures released earlier this year showed more than one in five energy customers are now with smaller suppliers.