Energy regulator Ofgem has proposed that the Big Six energy firms should be investigated by the Competition and Markets Authority.
In an open letter to Britain's main energy suppliers, the regulator's chief executive officer Dermot Nolan said they were concerned that companies were "not passing on savings to customers".
He said: "A concern that savings were not being passed on to customers was one of the reasons why we have proposed a referral of the energy market to the Competition and Markets Authority for investigation."
More top news
The five-year-old was bitten on the face and had to undergo surgery to her eyelid after the dog attacked her at an event in West Yorkshire.
People who voted remain in the EU referendum have gathered at a protest in London to ask the government to make a plan.
Istanbul's busiest airport has been targeted in a deadly gun and bomb attack - here's what we know so far.