Telecom group MTN confirmed on Monday it was fined $5.2 billion by the Nigerian regulator for failing to disconnect subscribers with unregistered and incomplete SIM cards at its division in Africa's biggest economy, Reuters reports.
Shares in MTN were down more than 10 percent after the fine was announced. They were down 14.25 percent at 166.99 rand by 1428 GMT.
Africa's largest mobile operator said in a statement that the Nigerian Communications Commission (NCC) imposed the fine on MTN Nigeria and said it related to the timing of the disconnection of 5.1 million subscribers in August and September.
"MTN Nigeria is currently in discussions with the NCC to resolve the matter in recognition of the circumstances that prevailed with regard to these subscribers," the company said.
The NCC said all other telecom firms operating in Nigeria complied with the directive except MTN, Tony Ojobo NCC's director of public affairs said.
The regulator said MTN, whose biggest market is Nigeria, had not denied the infraction and that its enforcement team deactivated the unregistered lines to ensure compliance.
"We had a directive that was given to all the networks to deactivate all unregistered SIM card on their networks and they had enough time to do that," Ojobo said.
"We came out with all guidelines and the fine was supposed to be 200,000 naira per SIM card."
MTN reported a slowdown in quarterly subscriber growth last week citing stiff competition and tougher regulation in Nigeria.
"Whenever the Nigerian regulator steps up enforcement, MTN takes a hammering," Dobek Pater, a telecoms analyst at Africa Analysis based in Johannesburg, said.
Nigeria contributes around a third of MTN's sales, he said.