The Economic and Financial Crimes Commission (EFCC) may open an investigation into the financial dealings of MTN Nigeria, the largest GSM service provider in the country with market share of over 40 percent , a source within the Commission has disclosed.

Sources at the EFCC, Awolowo Road, Lagos office hinted our correspondent on Monday that, the investigation was sequel a petition filed by an undisclosed Nigerian government agency against MTN.

The EFCC source said the Commission would be prying into the dealings and the books of the telco with a view to establishing if it was conforming to financial rules. The investigation would also zero in on whether the non-disconnection of the controversial SIMs gave MTN Nigeria any undue market share advantage over other network operators.

The Nigerian Communications Commission, the nation’s telecommunications regulator, had slammed  MTN Nigeria a hefty fine  of N1.4 trillion for failing to disconnect subscribers with unregistered SIM cards bought before January 2012.

Phone companies were told to register their existing customers’ SIM cards, which MTN Nigeria – the biggest telecommunications firm in Nigeria – has failed to do, reports the BBC.

The fine is the largest in the history of telecom infringements in Nigeria and may redefine the relationships between telecommunications operators and the regulator.

Nigerians continue to complain about poor services from mobile network operators and blame the regulators for not doing enough to solve the problem.

A senior executive at MTN Nigeria said the company will issue a statement on the fine.

Meanwhile, shares in South Africa’s MTN dropped more than 5 per cent on Monday after reports in Nigeria said the mobile phone firm’s Nigerian unit could face a fine of as much as $5.2 billion, traders said.

MTN declined to comment on the matter.

“The report is definitely the reason why it is moving. We just don’t know how credible it is,” said Petri Redelinghuys, a trader at Inkhunzi Investments in Johannesburg.

A second trader confirmed the share was falling as a result of the report.

Chris Maroleng, the Johannesburg-based spokesman for Africa’s largest mobile operator, declined to comment.