Firmus Energy discovers business gas bill error
Gas supplier Firmus Energy has discovered an error with bills sent to 37 business customers, one of whom was left facing a charge of £8,500.
Banbridge-based Franklins Group was told it had been undercharged for gas for the past 44 months.
But Firmus has subsequently apologised and said none of customers will be asked for the money.
In a statement to the BBC it said: "The issue has been traced to human error in inputting information and has now been rectified."
It said less than 1% of more than 3,000 of its business customers had been affected by the problem, which was detected in a recent internal audit.
The Franklins Group had been told the fault in the calculation of its bills was based on meter settings.
"It came as a bit of a hit. We always assumed the bills we were getting were correct," said Adrian Markey, the company's financial controller.
"News was being broken face-to-face by someone who came to check the meter. It was his job to deliver the Christmas present."
It had not been told the charge would subsequently be waived, that only emerged after the BBC contacted Firmus.
"For those customers who have been undercharged there will be no impact and credits will be applied this month for any customers who have been overcharged," said Firmus.
"No other customers are affected. We sincerely apologise to those customers affected for any inconvenience caused."