Hundreds of customers of Boost Energy have been hit with unexpected bills after the gas and electricity firm accidentally undercharged them.
The issue happened after defective smart meters gave lower energy readings by mistake.
Boost Energy is part of OVO Energy and supplies gas and electricity to roughly 200,000 pay-as-you-go customers.
Prepayment households are typically those who are struggling or on low income.
An email seen by The Mail on Sunday reads: “Unfortunately, when our prices went up in October, your meter wasn't updated due to a technical problem.
“This means we were charging you less than we should have for a short period of time. We're sorry about this.”
It continues: “What happens next: you will need to pay back the difference.”
The newspaper claims some households have been given additional bills of up to £700, with customers complaining about the issue on social media.
One said: “I'm at my wits' end. I can barely afford energy prices as it is.”
Another said: “It's a total disgrace and you can't speak to anyone concerned when you have a problem.”
Boost is offering some customers a discount of 10% and said those who are deemed the most financially vulnerable won’t have to pay the money back.
A spokesman said: “A small number of customers were paying too little for a short period due to a technical issue.
“We're here to help any customers who would like additional support with an affordable plan.”
The Mail on Sunday claims Ofgem is now investigating the incident.
It comes after rival Octopus Energy accidentally charged customers just 0.1% of the amount of gas they used during winter - but households didn't have to pay the money back.
The energy firm said the 99.9% undercharge was a result of an "obscure technical error" relating to customers with old gas meters which run with imperial measurements rather than metric ones.
Energy bills are currently dictated by the Energy Price Guarantee (EPG) which is set at £2,500 a year for the typical household.
But the EPG does not set an absolute limit on how much you can pay for energy.
Instead, it sets a limit on what you can be charged for units of gas and electricity, plus the daily standing charge.
The headline figures are used to illustrate what someone with typical usage can expect to pay over a year.
This means you could can end up paying more or less, depending on how much energy you use.
