A probe into SSE's processes for switching customers on to pre-payment meters has been launched by an energy regulator following concerns over whether they were treated fairly.
Ofgem said an investigation has been launched after worries about SSE's approach to switching its customers to pre-payment meter tariffs, particularly those in potentially vulnerable situations.
The investigation will focus on whether SSE breached Ofgem's Standards of Conduct, which are aimed at ensuring suppliers treat customers fairly.
Ofgem emphasised that the opening of the investigation does not imply that it has made any findings about non-compliance.
It will look at whether the supplier breached conditions which require them to provide appropriate information and ensure consumers' ability to pay when suggesting alternative payment methods.
It said: "We are concerned that SSE may not have been treating its customers fairly when switching them to pre-payment meters (PPMs) particularly those in vulnerable situations."
Rules surrounding the supply of gas and electricity require customers to be offered a wide choice of payment methods for paying charges under a domestic supply contract.
Firms should take all reasonable steps to ascertain a customer's ability to pay and must take this into account when calculating instalments.
They must offer certain services when they become aware or have reason to believe that the customer is having or will have difficulty in paying all or part of their charges.
These services include being able to pay charges by deduction at source from social security benefits, to pay by regular instalments through means other than a PPM and by using a PPM.
A spokesman for SSE said: "We are committed to treating all our customers fairly and will be cooperating fully with Ofgem's investigation into this historic issue."