The energy regulator has turned its glare on Utility Warehouse to investigate how the Big Six challenger brand treats customers who are struggling to pay their bills.

Ofgem said it will investigate whether the supplier has breached the industry code for dealing with customers who are in debt.

The probe will focus on the whether Utility Warehouse has offered its indebted customers “appropriate repayment options”, and whether they are misusing warrants to force the installation of pre-pay meters to recover debts.

“The opening of this investigation does not imply that there’s been any non-compliance by the supplier,” the regulator said.

A spokesman for Utility Warehouse said the supplier is "surprised and disappointed" that Ofgem has decided to launch a formal investigation into its 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," he said.

"We will, of course, work alongside Ofgem to ensure that this investigation is concluded," the spokesman added.