MTN Group CEO, Sifiso Dabengwa, has resigned with immediate effect after the company’s Nigerian operations was fined $5.2 billion by the industry regulator , the company announced early on Monday.

The Africa’s largest telecoms operator, with has 233 million subscribers in 22 countries in Africa, Asia and the Middle East, was two weeks ago slapped with the largest fine any company on the continent has ever faced for allowing 5.1 million subscribers in Nigeria to continue using unregistered SIM cards after a deactivation deadline expired.

“Due to the most unfortunate prevailing circumstances occurring at MTN Nigeria, I, in the interest of the company and its shareholders, have tendered my resignation with immediate effect,” said Dabengwa in a statement.

Dabengwa has been chief executive of MTN since 2011.

The company has appointed  Phuthuma Nhleko as executive chairman in a temporary capacity. He has chaired the group in a non-executive capacity for the past two-and-a-half years, Business Report indicated.

Nigeria is MTN’s leading market with nearly 63 million subscribers as of September, making it the largest operator on the West African Nation.

Shares in the Johannesburg-based telecom plunged more than 20 percent, wiping out nearly $4.2 billion off its market value, after the fine was announced and have remained subdued as uncertainty over the future of the company’s operations in Nigeria remain a concern.

The Johannesburg Stock Exchange said last week it was investigating MTN for “possible insider trading” before the South African telecoms firm announced that it had been hit by the fine in Nigeria.

Questions have been raised about the firm’s governance as it failed to disclose the fine in a timely nature to it shareholders.

MTN is in talks with the Nigerian Communication Commission over the fine, which is due on November 16.

Ratings agencies Moody’s and Fitch lowered MTN’s credit rating outlook to “negative” in October, flagging the risk of significant cash outflow and the likely damage to the Nigerian business due to lengthy talks, Reuters reported.