MTN has been accused of illegally repatriating about $14bn from Nigeria, the mobile network operator’s largest market.

The news sent the share price down about 3% to close at R119.77. According to a Bloomberg report, Nigeria’s senate said on Tuesday it would investigate the claim thoroughly.

Africa’s biggest wireless carrier by sales is accused of repatriating the funds over 10 years starting in 2006, according to Dino Melaye, the politician who tabled the motion. The four banks involved in the alleged illegal transfers are Citigroup, Standard Chartered, and Nigerian lenders Stanbic IBTC Holdings and Diamond Bank.

MTN and Standard Bank Group, which controls Stanbic, declined to comment.

But analysts expressed doubts about the veracity of the claims and how this had gone undetected for such a long time, especially since the Nigerian authorities had tight controls on foreign exchange.

Mergence Investment Managers’ portfolio manager Peter Takaendesa said it was difficult to believe that anyone could use top banks that were heavily regulated domestically and internationally to move cash out of the country for such a long period without complying with domestic requirements.

“It would also be quite a surprise if the allegations are proven correct in the Nigerian market, given the Central Bank of Nigeria has tightly controlled the foreign exchange market for quite a long time under the previous foreign exchange policy,” he said.

The accusation comes a few months after MTN agreed to pay a 330-billion naira (about $1bn) fine to the Nigerian government and to list its local unit on the country’s stock exchange. That penalty was levied for missing a deadline to disconnect 5.1-million unregistered customers in the country.

The initial fine was $5.2bn. MTN’s share price has fallen more than 37% since the fine was first reported in October.

My Wealth Investments CEO Devin Shutte said: “Unfortunately, MTN seems to be the gift that keeps taking from shareholders at the moment.”

The market needed clarity on the details of these alleged transactions, he said, noting, however, “We can see by the market reaction that many shareholders are not taking chances on what has already been a rough ride with the company of late.”

The Nigerian government is under pressure to raise revenue to improve the ailing economy. There is speculation that some politicians in the country were unhappy with the reduction of the fine levied on MTN, and the latest disclosure could be motivated by similar thinking.

“The unfortunate reality is that MTN has made a concerted effort to improve its corporate governance recently, but this latest revelation indicates that there might still be a few skeletons in the closet that need to be dealt with,” he said.

Takaendesa said the continuing negative news flow from Nigeria reflected badly on MTN, as well as on Nigeria as an investment destination.

“It is very unlikely that MTN would have moved all these funds to SA, given foreign exchange shortages in Nigeria,” he said.