It will look into whether customers’ energy bills were above the permitted level of the price cap between May and September 2019 – the month when the regulator appointed Utilita to take on the customers of failed energy supplier Eversmart Energy.
Under the last resort process, around 39,000 domestic customers and a very small number of business customers were switched to Utilita.
The PPM price cap, currently set at £1,242 per year for a dual fuel customer with typical use, came into force in April 2017.
The cap is temporary but the Competition and Markets Authority (CMA) has suggested Ofgem should continue to protect PPM customers beyond 2020.
It applies to PPM customers on a non-fixed deal and without an interoperable smart meter.
The regulator said: “The opening of this investigation does not imply that we have made any findings about non-compliance by Utilita.”
A spokesperson from Utilita said: “We are aware Ofgem has opened an investigation and will assist the regulator in every way we can.”