The energy watchdog Ofgem has said that the Big Six should pass on savings from wholesale prices to customers as soon as it is possible.
In an open letter, the regulator's chief executive officer, Dermot Nolan, said: "In a competitive market, I would expect the threat of losing market share to encourage suppliers to pass on sustained reductions in wholesale costs as savings to consumers as soon as possible.
"If that is not happening, it could be seen as further evidence that competition is not working for consumers as well as it should be."
More top news
Temperatures set to slide
The announcement was hailed as a "momentous step" that will help families facing financial stress as they come to terms with their loss.
Pop Idol's Michelle McManus has joined 10 choirs to record a charity single that they hope will make Christmas Number One.