Wales and West Utilities is to be investigated by Ofgem after "discrepancies" were found in the energy supplier's records.
The Daily Telegraph reported that the company, which provides gas supplies to more than seven million homes, will have its records and recording assessed by the watchdog.
Ofgem will examine the energy firm's mains replacement programme to understand how far any cases of misreporting have reached.
Wales and West said in a statement that it discovered the errors itself and would work with Ofgem to thoroughly investigate these.
Graham Edwards, chief executive of the company, added: "Proper recording of regulatory information is fundamental to WWU's activities and so we promptly reported our findings to Ofgem and took immediate steps to rectify the situation."
The regulator is working to make energy bills fairer and more transparent for UK customers.
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