The country's poorest families could face paying another £80 a year for their energy bills from April.
Power experts and charities said they believe industry regulator Ofgem will raise the annual price cap for those on pre-payment meters - which tend to be in households already struggling to make ends meet.
It would take it from an average of £1,070 to £1,151.
The £81 increase, predicted by energy consultants Cornwall Insight, would virtually wipe out a £95 annual saving announced in October, the Mirror reports.
Adam Scorer, of the charity National Energy Action, said other customers can avoid higher bills by switching supplier or tariffs - but pre-payment meter users may not have other options.
He said: "This will hit millions of pre-payment customers who are already facing a tough winter. They should be better protected from price rises.”
Robert Buckley, of Cornwall Insight, said rises are driven by higher wholesale energy costs and unless they fall, it could see the cap rise by another £40 or £50 next winter.
Cornwall believes Ofgem will raise the cap by £66 for 11 million customers on standard tariffs.
They also face a £21 a year charge to cover others missing repayments due to the Covid-19 crisis.
Ofgem declined to comment.
