Energy regulator Ofgem called for speedier action from suppliers to slash unfair pricing from energy bills.
The regulator said firms were on track to slash more than £500 million from bills, but it is not happy with the pace of progress.
Ofgem wants faster action from companies to end the premiums paid by poorer households using pre-payment meters for their energy needs.
The regulator said that since its probe began, premiums paid by customers including pre-payment meter users had fallen by more than £300 million.
At the beginning of the year, pre-payment meter customers were paying an average of £125 more than direct debit customers. Suppliers have also signalled at least a further £200 million in reductions for more than four million households who are not connected to mains gas and unable to gain the best deals.
But the regulator threatened firms with a full referral to the Competition Commission unless they moved more quickly to combat unfair pricing.
Ofgem published the initial findings of its energy market probe in October. Chief executive Alistair Buchanan said: "We've seen encouraging signs since the end of our initial investigation but we demand more and quicker action for those customers currently losing out.
"And we are about to consult on new rules to end unfair pricing in future. If we are not satisfied with the suppliers' responses we can, should we choose, go to the Competition Commission."
In January the regulator will launch a six-week consultation on its proposals to change the suppliers' licences to ban unfair prices and ensure consumer interests are more fully protected. "If sufficient progress is not made following consultation, the authority will consider further action," Ofgem added.
Members of the "big six" UK energy firms have made announcements over lower tariffs in recent weeks such as E.ON, which cut bills for customers in the East Midlands, East of England and North West without access to the mains supply.
