Shares in MTN Group, Africa’s biggest cellphone operator, fell sharply at the opening on Friday, following Thursday’s profit warning.

The shares, which fell 13% in early trade, continued to fall and were quoted almost 19% lower at R153.70 shortly before noon.

MTN said on Thursday that it expected headline earnings per share to fall about 20% for the year to December, due to a poor performance in Nigeria.

MTN faces a $3.9bn fine from Nigerian regulators for missing the deadline to disconnect 5.1-million subscribers who were not registered as required by the law. The group is seeking an out-of-court settlement.

Nigeria accounts for 37% of MTN’s revenue. Last month MTN Nigeria confirmed it would report profit of about $995m — a fraction of even the reduced fine. The fine initially was $5.2bn but was reduced by 25%. MTN Nigeria CEO Ferdi Moolman said at the time that the reduced fine could bankrupt the company, as it represented 95% of its annual turnover

The fine is also more than twice the group’s annual average capital spending over the past five years.

MTN said, given the discussions, "There remains some uncertainty as to the final quantum of the Nigerian fine, should an out-of-court settlement be reached."

It would issue a further trading statement "as soon as there is a reasonable degree of certainty" about the likely effect of the Nigerian troubles on its earnings.