Energy customers on pre-payment meters have been paying an average of £226 a year more than they would have done on the cheapest direct debit tariff.
That’s according to charity Citizens Advice.
They added the gap between the cheapest and most expensive tariffs widened by up to a third in six months.
Co-Operative Energy, which had the widest gap of £405, responded by saying it was cutting its pre-payment rates.
It comes ahead of the results of an inquiry into the whole energy industry.
The Competition and Markets Authority (CMA) began its investigation a year ago and is due to report on Tuesday.
Gillian Guy, the chief executive of Citizens Advice said: “The CMA investigation provides an opportunity to overhaul the energy industry and make it fairer.”
“Giving pre-payment meter customers a better deal is a crucial part of this.”
The charity went on to say people with pre-payments meters are more likely to be on a low income and 80% of those getting installed are already in debt.
Those who fall behind with payments can be forced to have them, if their supplier obtains a court order, it said.