The energy regulator said it was responding to concerns about the Perth-based company’s approach to switching customers to pre-payment tariffs.

The probe will focus on the impact of the process on those in potentially vulnerable situations.

Ofgem will consider whether SSE breached its Standards of Conduct, which aim to ensure suppliers treat customers fairly.

It will examine whether the company breached licence conditions, which require it to provide appropriate information, and ensure a consumers’ ability to pay, when suggesting alternative payment methods.

Craig Salter, energy spokesman for the CAS Consumer Futures unit, said: “Suppliers must always ensure that customers in debt receive the levels of support and protection that they need.

“We know that consumers using PPMs have less access to lower-cost tariffs than those using other payment methods, so moving consumers to PPMs should be a last resort – particularly because PPMs are already more common in Scotland.

“It is therefore appropriate that Ofgem carries out this investigation in order to ensure that SSE’s vulnerable customers are treated fairly.”

SSE described the basis for the investigation as a “historic issue”.

A spokesman for the energy provider said: “We are committed to treating all our customers fairly and will be cooperating fully with Ofgem’s investigation into this historic issue.”