MTN Group shares slumped the most in three months after Nigerian lawmakers raised new allegations about the wireless carrier, this time accusing the company of illegally moving almost US$14bn out of its largest market.

Nigeria’s senate will thoroughly investigate the claim, it said on its Twitter account on Tuesday.

The Johannesburg-based company, 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 made the motion. The four banks involved in the alleged illegal transfers are Citigroup, Standard Chartered and Nigerian lenders Stanbic IBTC Holdings and Diamond Bank.

Representatives for MTN and two of the banks weren’t immediately available to comment. Citigroup and Johannesburg-based Standard Bank, which controls Stanbic, declined to comment.

MTN shares fell as much as 4,4%, what would be the biggest fall on a closing basis since 27 June, and traded 3,4% lower at R119,77 as of 1.32pm in Johannesburg. That values the company at R221bn.

The accusation comes a little over three months after MTN agreed to pay a 330bn naira ($1bn; R13,5bn) fine in cash to the Nigerian government and list its local unit on the country’s stock exchange after about eight months of negotiations.

That penalty was levied for missing a deadline to disconnect 5,1m customers unregistered in the country, which is battling an Islamic insurgency. The stock has fallen more than 37% since the fine was first reported in October, allowing crosstown rival Vodacom Group to overtake the company in terms of market valuation.

“It’s not a good thing for MTN that there is a lot of noise being made again around corporate governance,” Sasha Naryshkine, an analyst at Johannesburg-based money manager Vestact, said by phone. “It’s difficult to move money out of the country illegally as it has to be moved through the central bank.”

MTN shares had fallen earlier on Tuesday after the Nigerian telecommunications regulator blocked the carrier’s attempt to take over Internet spectrum owned by closely held Visafone, which it had agreed to buy in January. MTN is trying to add capacity to extend Internet access in Nigeria and South Africa as revenue from voice services declines.