MTN has received a $2bn tax bill from Nigeria, it said on Tuesday, the latest demand by authorities in the company’s most lucrative but increasingly problematic market

The news sent MTN’s share price at the Joburg Stock Exchange as much as 7.4% lower in afternoon trade: at 1.45pm, MTN had fallen 6.67% to R81.05. The company has now lost 31% of its value since the beginning of August.

The announcement of the tax bill comes days after the West African country’s central bank ordered MTN’s Lagos-based unit to hand over $8.1bn that it said was illegally sent abroad.

In a statement, MTN disclosed that it had been in talks with Nigeria’s attorney-general about an investigation into tax compliance by the mobile operator.

“In this process, [the attorney-general’s] office made a high-level calculation that MTN Nigeria should have paid approximately $2bn in taxes relating to the importation of foreign equipment and payments to foreign suppliers over the last 10 years,” MTN said.

MTN said its total payment of about $700m fully settled the amount owing under the taxes in question.