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 High Court will hear a legal challenge to Labour's decision to guarantee Jeremy Corbyn a place on the party's leadership ballot.
Russian Foreign Minister Sergei Lavrov brushed aside accusations that Moscow was behind the hacking of internal Democratic Party memos.
The First Lady gave an enthusiastic backing for the Democratic presidential frontrunner in a speech at the Democratic National Convention.