An East Yorkshire council has agreed pay £17,000 to settle its debts after failing to pay a gas bill for 17 years.
It found that it did not have a deal with any energy supplier.
At the time, chair of the council's personnel committee, Denis Healy, said it was "completely unacceptable" that the problem had gone unnoticed for so long.
In February, the council said it had set aside around £35,000 to clear the debt as it waited for confirmation of the final amount it owed.
The final bill was calculated using OFGEM rates, and the council's finance committee said it was a "fair figure."
An internal inquiry has now opened to find out how the oversight happened.