At least 1.5 million people displaced by the Islamist insurgency in north-east Nigeria may not be able to vote in elections if the law is not changed, an electoral official has told the BBC.
Discrepancies in the law needed to be resolved in "very good time" or people could be disenfranchised, he added.
Ex-military ruler Muhammadu Buhari will challenge President Goodluck Jonathan in the February election.
Boko Haram's insurgency has mainly affected opposition strongholds.
Last year, Mr Jonathan imposed a state of emergency in the north-eastern states of Adamawa, Borno and Yobe in a bid to curb the insurgency.
However, Boko Haram has stepped up attacks since then and has declared an Islamic state in areas it controls.'Staggered voting'
BBC Nigeria reporter Will Ross says it is not clear whether the elections will take place at all in states under emergency rule.
But the Independent National Electoral Commission (Inec) said it was determined to ensure that the elections took place in all parts of the country.
The vote could be held on a staggered basis and areas could be secured with "proper deployment" of the security forces, Inec spokesman Nick Dazzang told BBC Focus on Africa.
Inec was distributing voter cards to displaced people, many of whom were living in camps, but discrepancies in Nigeria's Electoral Act needed to be "reconciled", he added.
It stated that people could "transfer" their registration to where they were living but it also stated that they needed to vote where they were registered, Mr Dazzang said.
"We are concerned that the way the law is structured now, unless it is amended in very good time, some of them will be disenfranchised," he told BBC Focus on Africa.
Our reporter says the election is expected to be one of the most keenly fought since the end of military rule in 1999 - and that has prompted some warnings of potential violence.
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