TORONTO -- First Quantum Minerals Ltd (FM.TO) says the Zambian government has issued it a $10.4-billion bill for underpayment of customs duties.

The company says the letter from the Zambia Revenue Authority has assessed the costs on a combination of import duties, penalties, and interest on consumables and spare parts.

First Quantum says it "unequivocally refutes" the assessment, that the bill does not have "any discernable basis of calculation," and that it will continue to work with the authority to resolve the issue.

Shares of First Quantum fell as much as 9 per cent when markets opened, after dropping more than 12 per cent to $18 before it was halted on the Toronto Stock Exchange on Tuesday.

First Quantum operates two mines in Zambia, including the Kansanshi mine that it says is the largest copper mine in Africa.