The Warm This Winter campaign obtained data indicating that 274 fixed tariffs set energy costs higher than the cap.
As a result, while most households witnessed slight bill reductions starting 1st July, those affected face significantly higher expenses.
On average, bills will surge by approximately £500 per year, with some customers paying almost 2.5 times the Ofgem limit, according to the report, which estimated that there are nearly 1.5 million customers affected by these tariffs.
While a few customers can switch without penalties, others may face exit fees ranging from £50 to £400.
Simon Francis, Co-ordinator of the End Fuel Poverty Coalition, commented: “It turns out they’ve been taken for a ride by energy firms who may now be charging them more for their energy than people on the Ofgem-fixed standard variable tariff.
“Energy firms must work immediately to end this discrepancy and bring all tariffs into line with the Ofgem price cap or waive exit fees for these customers. If energy firms won’t act, the government must reduce the Energy Price Guarantee to be in line with the Ofgem price cap.”