(Adds MTN's chief executive comment)
By Chijioke Ohuocha and Alexis Akwagyiram
ABUJA/LAGOS, Sept 1 (Reuters) - MTN Group's plan to list its Nigerian unit in an initial public offering this year is under threat after the central bank ordered the South African telecoms giant to hand over $8.1 billion allegedly sent abroad illegally, sources said.
Nigeria's central bank alleged that MTN used improperly issued certificates to transfer funds out of Nigeria after the telecoms giant converted shareholder loans in its Nigerian unit to preference shares in 2007. MTN denies the allegations.
The monetary authority said MTN's banks had failed to verify that the telecoms group had met all the country's foreign exchange regulations.
The money is more than half of MTN's market capitalisation. MTN Group shares plunged as much as 25 percent to a nine year low on Thursday after the allegations were levelled by Nigeria's financial regulator.
Africa's biggest telecoms company had previously said it aimed to list this year in its largest market, the continent's most populous country of 190 million people, as it seeks to expand mobile services to boost margins and access new revenue streams.
But three people familiar with the matter said the scale of the central bank's demand affects the market conditions, casting into doubt the likelihood that the process would be completed by the end of the year, if at all.
"It will affect our timelines and key events," said one of the people familiar with the matter, who did not want to be named.
The person said the development meant the IPO prospectus that had been prepared would have to be altered.
In an emailed statement, MTN Group Chief Executive Rob Shuter said: "The implementation of our IPO has always been subject to satisfactory market conditions and this event will potentially make it complicated for us to conclude the process.
"Despite this, we have instructed our advisors and our teams on the ground to continue at full pace," he said.
Earlier this year Shuter told Reuters plans for the Nigerian listing were at an advanced stage and it should be concluded this year if market conditions are appropriate.
The demand by Nigeria's central bank is the latest setback for MTN in Nigeria, coming two years after it agreed to pay more than $1 billion to end a dispute in Nigeria over unregistered SIM cards.
Nigerian banks are set to hold a regular meeting soon at which they will discuss the payment and how to "engage government and regulators to resolve the issue," according to Access Bank Chief Executive Herbert Wigwe.
Additional reporting by Tanisha Heiberg in Johannesburg Editing by Alexandra Hudson and Ros Russell