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
Son of barrister hired by Lord Janner told ITV News his father once believed peer would have to face charges over child abuse allegations.
Low pressure brings cooler, showery conditions over the next week
100-year-old tourist mugged by stranger after he fell getting off a bus as he returned to Gloucester where he used to live.