Which? executive director Richard Lloyd, said:
“This is yet another shameful example of energy companies failing their customers and one that has hit some of the very people who can afford it least.
“Suppliers need to fix this fault as soon as possible and fully reimburse – with interest – the hundreds of thousands of people who have been overcharged.
“We urgently need major reforms to fix this market and increase the low levels of trust we consistently see in energy firms.”