Around one million households are set to get energy refunds after 18 suppliers overcharged customers after they switched to a better tariff.
Energy regulator Ofgem said 18 energy suppliers will pay £10.4million back to consumers for contract renewal failures.
It said they failed to correctly protect customers' tariff prices when they decided to switch supplier or tariff - reports Mirror Online.
Ofgem said more than one million customers were collectively overcharged £7.2 million after their suppliers failed to follow price protection rules, which safeguard a customer's tariff price when they decide to either switch suppliers or deals after a price increase.
It said several suppliers self-reported the issue to the regulator, which then prompted all firms to assess their practices, which revealed failures among 18 suppliers between 2013 and 2020.
The firms have agreed to refund all those affected and, in some cases, make additional good will payments to the tune of £10.4 million in total, Ofgem said.
Anna Rossington, interim director of retail at Ofgem said: "Customers should have confidence in switching and not be overcharged when doing so.
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"This case sends a strong message to all suppliers that Ofgem will intervene where customers are overcharged and ensure that no supplier benefits from non-compliance.
"It also shows that, where appropriate, Ofgem is prepared to work with suppliers who have failed to comply with the rules, but who are willing to self-report issues and put things right for their customers."
