Between 2012 and early 2020, a system error meant customers with an outstanding debit balance of £40 or more owed to iSupply were blocked from switching supplier.

However, switches can only be prevented if the money owed has been billed at least 28 days before the switching request.

Energy regulator Ofgem said that as a result, 22,863 customers were blocked from a move.

As such, the supplier is to pay £406,224.81 (an average of £21.65 per customer) in refunds as well as a customer goodwill payment of £114,315.

The total redress stands at £520,539.81. Where customers can’t be contacted or cheques aren’t cashed, the money will go into the Voluntary Redress Fund.

Ofgem confirms this is on top of the £1.5m paid out in June after 115,000 customers were impacted by historic failings and the £1.5m it was forced to pay in December 2019 relating to overcharging issues.

Ofgem said it continues to engage closely with iSupply as it leaves the market in 2020. In March, iSupply’s parent company, Vattenfall, reached an agreement with EDF to transfer energy contracts to them.

YourMoney.com has approached iSupply and EDF for comment.