Utility Warehouse is being investigated by Ofgem over the way it manages its customers that get into debt - particularly on whether it puts in place appropriate repayment options.
The energy regulator will examine if the supplier does enough to try to contact customers in or at risk of debt, and establishes manageable repayment plans based on their ability to pay - required by gas and electricity supply licence conditions.
It will also investigate if Utility Warehouse is installing prepayment meters appropriately and proportionately, as a means of recovering debt - particularly when it installs them under a court-issued warrant, an area Ofgem had recently clamped down on.
A spokesperson for Utility Warehouse said: "Utility Warehouse is surprised and disappointed that Ofgem has decided to launch a formal investigation into some of our processes.
"An independent audit of these processes late in 2017 gave an overall positive report with a number of best practice recommendations now having been fully implemented."We will, of course, work alongside Ofgem to ensure that this investigation is concluded."
The latest figures from Ofgem show that, at the end of June 2017, there were around 980,000 gas and 1.5 million electricity accounts either currently repaying a debt, or in debt but not yet repaying.