The UK's energy market is to face a full investigation by the Competition and Markets Authority (CMA), the regulator Ofgem has announced.
The probe will last 18 months and is expected to include a look at the profits of the six largest suppliers.
Ofgem said in March it intended to refer the industry to the CMA.
"This will help build consumer trust and confidence in the energy market." said Ofgem chief executive Dermot Nolan.
"There is near-unanimous support for a referral and the CMA investigation offers an important opportunity to clear the air."
In March an Ofgem report questioned the effectiveness of competition in the market and recommended a full inquiry. It was then required by law to consult other interested parties before making a final referral.
Ofgem also wants the CMA to examine whether there is sufficient competition between the large energy providers, and whether consumers who do not switch supplier are being set higher prices.