More than one million people were overcharged by energy firms during the process of switching suppliers, the regulator Ofgem has said.
When a customer decides to switch, the rules state they should be protected from price rises while moving – which could take up to three weeks.
Eighteen suppliers made errors leading to overcharging of £7.2m.
They are refunding the money and making extra payments in some cases, taking the compensation total to £10.4m.
The biggest names in the sector – including British Gas, E.On, EDF, Npower, Scottish Power, SSE and Octopus – made the mistakes.
They failed to protect customers who switched tariffs or suppliers after price rises were set between 2013 and 2020.
Although the average amount overcharged was only £6.27, the sums would be higher for those with the biggest bills, and Ofgem said companies did not have the “adequate arrangements” in place to prevent overcharging.
Several companies had self-reported the issue to Ofgem, which then required all suppliers to check their records.
Anna Rossington, interim director of retail at Ofgem said: “Customers should have confidence in switching and not be overcharged when doing so.
“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.”
In changes made last year by Ofgem, customers receive automatic compensation of £30 if their switch to a new provider goes wrong.
Payments are made if the switch is not completed within 15 working days. A mistaken switch or a failure by the old supplier to provide a final bill within six weeks also qualify for compensation.