Nigeria's telecommunications regulator has fined South African mobile giant MTN $5.2 billion for missing a deadline to disconnect unregistered SIM cards, the company announced on Monday.
The penalty saw shares in Africa's largest telecommunications company crash more than 12 percent to 167 rand on the Johannesburg Stock Exchange, the biggest fall the firm has suffered in a day since November 1998.
"This fine relates to the timing of the disconnection of 5.1 million MTN Nigeria subscribers," MTN said in a statement.
The Nigerian Communications Commission fined the company 200,000 naira (about $1,000) for each unregistered SIM.
Early in August the commission issued a directive to mobile telecoms companies operating in Nigeria to deactivate all unregistered SIM cards within seven days or face severe sanctions.
MTN Nigeria said it is now engaged in talks with the commission in a bid to resolve the matter.
Nigeria, Africa's most populous country, is MTN group's largest market where it had over 62.8 million subscribers by the second quarter of this year.