Utilita Energy is under investigation into whether it overcharged customers covered by the pre-payment meter (PPM) price cap.
Ofgem announced this morning (9 January) that it was investigating whether customers were charged above the permitted level of the cap between May and September last year.
The PPM price cap is temporary and came into effect on 1 April 2017. It applies to PPM customers on a non-fixed deal and without an interoperable smart meter.
Suppliers can price to the level of the cap or below it, but cannot charge more.
The regulator stressed that the opening of the investigation does not imply that it has made any findings about non-compliance by Utilita.
Utilita was formed in 2003 and is one of the larger challenger brands.
It has recently taken on thousands of customers through the supplier of last resort (SoLR) process as fellow challenger brands have ceased trading and exited the market.
Last year it was named as SoLR for Our Power and Eversmart Energy.