Up to 100,000 mobile-phone subscribers are being overcharged by O2 for roaming calls made when they travel abroad, the firm admitted last night.
O2 post-paid customers making or receiving calls since the end of March are being affected by the overcharging, which in some cases amounts to an additional €1 per minute.
The company became aware of the problem in April, and appointed a team headed by its technology director, Mr Oliver Coughlan, to tackle it. But O2 admitted last night that the problem has not been fixed, and some consumers are still being charged too much.
The firm also confirmed that it had made no effort to inform individual customers or the media about the overcharging since it became aware of the problem.
An O2 spokeswoman said the problem was complex but should be sorted out within 24 to 48 hours. She said the firm was beginning to apply credit to affected customer bills to make up for the overcharging.
She could give no indication of total value of the overcharging by O2 or the exact numbers of post-paid customers affected by the overcharging.
O2 Ireland has about 15 separate billing cycles for customers each month, which means that it is still not clear how many customers have been overcharged.
Post-paid customers choose to have a contract and bill with O2 rather than paying for credit before using it.
The firm, which is the second- biggest mobile-phone operator in the Republic, has about 380,000 post-paid customers and a further one million pre-pay mobile subscribers.
"To be absolutely sure we repay everyone we are now applying credit to all post-paid customers who have travelled abroad," the spokeswoman said.
"But because of the complexity of roaming charges we've got to monitor customers' bills very closely to detect it."
She said all customers should be repaid by the end of July.
The error occurred because of a billing system problem at O2 Ireland, whereby customers were not offered a new preferential tariff introduced recently by O2.
An O2 spokeswoman said last night that some customers had been charged rates above its standard roaming tariffs set at 59, 79 and 99 cent. She admitted that in some cases they might have been charged rates up to €1 per minute more than they should have while making or receiving calls when abroad.
Roaming charges have proved controversial in the Republic and throughout Europe because of the hundreds of tariffs that firms set for different types of calls. Consumer associations have regularly expressed concern that the fees are not transparent for the average consumer.