He added that he had “since made significant changes to prevent this from happening again.”

Prepaid customers, who are “often vulnerable”, were not informed they were being credited when they canceled their contract, Ofgem said.

The credit amounts to £4.7 million in total, but E.On is required to pay compensation of £6.6 million and has voluntarily agreed to pay £3.2 million.

Ofgem said the total payment of £14.5 million was “in recognition of the impact on its customers, many of whom may have faced financial difficulties”.

In cases where the customer cannot be identified, Ofgem said E.On would contribute to the Energy Industry Voluntary Redress Scheme, a charity which supports vulnerable energy customers.

E.On also agreed to cancel debts held by nearly 150,000 prepayment customers who canceled their accounts during the same period.

Ofgem said it was continuing its claims “against other suppliers over the same issue”.