Northern Ireland's water company has overcharged customers by up to £1.8 million, it has been revealed.
The government-owned organisation, which came under fire when thousands of homes lost water supply last Christmas, said nearly 1,300 customers were affected by the latest incident to hit the company.
Northern Ireland Water (NIW) blamed inaccurate records on the size of pipes serving metered customers for the overcharging, and revealed that other properties were undercharged by an estimated £350,000.
It said it will contact all the metered properties involved to settle the bills and said the error had not affected domestic properties. Most metered customers are businesses, but public buildings such as churches also have water meters.
Head of Customer Services Liam Mulholland said: "During the course of this review, a number of inaccuracies in the historical data have been discovered, affecting a small percentage of customers. This is a regrettable situation, and one which we apologise for.
"For some of these customers, there may be no impact on previous billing. However, there are likely to be some customers who have received incorrect bills as a result of the inaccuracies. This could lead to refunds or back payment depending on the circumstances but only affects the standing charge element of customer bills."
He said NIW would contact customers to "discuss the quickest and most appropriate means of addressing the situation".
Mr Mulholland said the review of the company's metered supply of water would be complete by Autumn 2011.
He added: "Customers can be assured that the accuracy and efficient management of metered customer accounts will continue to be one of our highest priorities."