Lagos - MTN Nigeria has been fined $5.2 billion for failing to disconnect 5.1 million unregistered cellphone subscribers.
The stock of the South African-based company fell 14 percent on the Johannesburg Stock Exchange on Monday's news.
The Nigerian Communications Commission confirmed the penalty based on 200,000 naira ($1,000) for each cellphone SIM card that had not been registered by its August deadline.
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MTN promised a statement later Monday. The $5.2 billion is at least two average years' profit for MTN Nigeria, by far the MTN Group's most lucrative subsidiary, and nearly three times the $1.8 billion that it has invested in Nigeria, according to the company's website.
MTN says it has about 62 million customers among Nigeria's 170 million people.