Ofgem is pushing the six main energy suppliers to strip unfair charges from their bills as the regulator targets £500 million of savings for consumers.
The energy watchdog said although the gas and electricity companies have so far taken off £300 million from customer bills, the reductions are not being made fast enough.
Ofgem said it is proposing to ban unfair pricing practices and may refer the market to the Competition Commission if progress is not made.
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."
Customers on pre-payment meters – often in disadvantaged households – have to pay significantly more than customers who can pay by direct debit and Ofgem said this group has seen savings as a result of the probe.
However, there are still four million households who are off the gas grid and therefore unable to get the best deals, according to the watchdog, and suppliers need to take action on this.
Ofgem believes these customers could be saving £200 million through fairer pricing.
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