MTN Group shares plunged as much as 23 per cent to a nine year low on Thursday, a day after Nigeria ordered the telecoms group to repatriate $8.1 billion illegally sent abroad.

At trading on the Johannesburg Stock Exchange, MTN shares were down 21.4 per cent at 84.35 rand, after touching 83 rand, a level last seen in 2009.

Nigeria’s central bank said the funds had been illegally moved abroad because the company’s bankers had failed to verify MTN had met all the foreign exchange regulations.

MTN denies the allegations.

The demand by Nigeria’s central bank is the latest setback for MTN in Nigeria, the South African group’s most lucrative but increasingly also its most problematic market.

It comes two years after MTN, Africa’s biggest telecoms company, agreed to pay more than 1 billion dollars to end a dispute in Nigeria over unregistered SIM cards.

The case underlines the risks of MTN’s strategy to operate in emerging markets, a move that has made it one of post-apartheid South Africa’s biggest commercial success stories, with operations in more than 20 countries.

The money is more than half of MTN’s market capitalisation, and analysts said the demand risked further undermining Nigeria’s efforts to shake off an image as a risky frontier market for international investors.

The crux of the allegation is that MTN used improperly issued certificates to convert shareholders’ loans in its Nigerian unit to preference shares in 2007.

Nigeria said that due to conversion, dividends paid by MTN Nigeria to the parent company between 2007 and 2015 – amounting to 8.1 billion dollars – were illegal, and should be returned.

MTN said in a statement, “No dividends have been declared or paid by MTN Nigeria other than pursuant to certificates of capital importation issued by our bankers and with the approval of the CBN (Central Bank of Nigeria) as required by law.”

“One wonders why this wasn’t brought to MTN’s attention years ago.

“You just can’t do business in an environment where these type of things are going to happen,” said Greg Davies of boutique investment house Cratos Capital in Johannesburg.

Nigeria’s central bank also fined four banks, including Standard Bank’s unit in Nigeria, Stanbic IBTC, for their role in moving the money out of Africa’s biggest economy.

On Wednesday, the central bank said it had fined Stanbic IBTC N1.8 billion (6 million dollars) for violations of foreign exchange transactions rules and ordered that it return 2.6 billion dollars it repatriated on behalf clients, including MTN.

Standard Bank, said in a statement, its unit was “not a beneficiary of any of the remittances made on behalf of clients and denies any imputation of malfeasance.”