Twelve million households who stay loyal to their energy supplier could overpay by £250 this year as their fixed-term deals default to more expensive standard rates.
Collectively, this "loyalty tax" will cost consumers more than £3bn in 2020.
The research, by Bulb, an energy company, showed several providers charge existing customers the "standard variable tariff" whereas new customers are signed up to cheaper fixed-term rates.
Once these teaser deals expires, customers end up paying 28pc more on average. The £250 surcharge is 53pc higher than 2019 as energy providers put the squeeze on their existing customers who are reluctant to change.
Bulb's figures showed that the average customer could cut their annual energy bill by £210, or nearly a quarter, by switching to an entirely different provider and taking advantage of cheaper introductory offers.
The energy watchdog Ofgem announced on Friday that the price cap on variable tariffs will be cut to £1,042 from October 1, its lowest level since it was introduced at the beginning of last year.
This will save more than 15 million households at least £84 per year and help to protect consumers from defaulting to a significantly higher tariff without realising.
However, despite the cap, changing providers regularly and taking advantage of introductory offers remains the best way for consumers to ensure they don't have to pay more than they need to.
Energy and Clean Growth minister Kwasi Kwarteng said: “Switching energy suppliers is the best way to save money on your energy bills, which is why we’re working with industry to make the process of switching quicker, easier and more transparent.
“Around a fifth of homeowners have switched their energy tariff in the past 12 months and I hope that number continues to rise.”
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Hayden Wood, of Bulb, said: “Energy companies should be rewarding their loyal customers, not punishing them. It’s frustrating to see that British households are still being misled by confusing deals, and charged, on average, 28pc more than they should be.”