INTERNATIONAL - MTN Group Ltd. plunged to a nine-year low after Nigeria’s central bank ordered the return of $8.1 billion it alleges Africa’s largest mobile-phone company illegally moved out of the country. 

The wireless carrier “strongly refutes” the claim.

The order comes about two years after Johannesburg-based MTN was first accused of repatriating dividends in contravention of foreign exchange laws. The company was cleared in a report issued in November 2017 following a probe commissioned by Nigerian lawmakers. MTN said in 2016 that its bankers had obtained central bank approvals before any dividends were issued.

“The re-emergence of these issues is regrettable as it damages investor confidence and, by extension, inhibits the growth and development of the Nigerian economy,” MTN said in a statement on Thursday. “We will engage with the relevant authorities and vigorously defend our position on this matter and provide further information when available.”

The shares slumped as much as 19 percent in early trade in Johannesburg to 86.99 rand, the lowest since March 2009.

The regulator fined Citigroup Inc., Standard Chartered Plc, Standard Bank Group Ltd.’s Stanbic IBTCand Diamond Bank Plc about 5.9 billion naira ($16 million) for helping to move the money, Nigeria’s central bank said in an emailed statement Wednesday. The banks and MTN were ordered to refund the cash.

The decision will come as a blow to MTN, because the company has already settled a separate, $1 billion fine in Nigeria for missing a deadline to disconnect unregistered customers. Negotiations over that penalty went on for almost a year and weighed heavily on the share price, which has never recovered. As part of the settlement, MTN agreed to list its local unit in Lagos and is planning to do so this year. Citigroup is working on that plan.

Standard Chartered received the largest penalty Wednesday for transferring the biggest amount of $3.4 billion, with Citigroup responsible for $1.7 billion. Stanbic IBTC is engaging with the central bank over the issue, it said in a statement on Thursday. The other lenders didn’t immediately comment.

Nigeria is MTN’s biggest market with more than 54 million customers, out of 221 million worldwide. The share price has slumped 21 percent this year to 107.34 rand, valuing the company at 202 billion rand ($14 billion), and touched eight-year lows earlier this month.