As a result of MTN’s woes in its most lucrative market, the company’s shares have dropped by about 20% in under a week.MTN says the office of Nigeria’s attorney general calculated that the company owes $2bn in taxes related to the imports of equipment and payments to suppliers done over the last 10 years.

The company disputes the amount and said it has already settled the bill with a $700m payment.MTN’s tax issues were revealed in a statement outlining the background of the $8.1bn Nigeria’s Central Bank said had been taken out of the country illegally.

The central bank says that if MTN returns the money, the mobile phone operator will receive a refund in Nigeria's currency, the naira.The South African company was previously hit with a $5bn fine by Nigeria’s telecommunications regulator in 2015 for failing to comply with a government order to disconnect five million unregistered numbers.

The fine was later cut to $1.7 bn.

MTN has over 50 million Nigerian customers and the country accounts for over 30% of the company’s business.