Zambia's President Rupiah Banda in parentage row
Zambia's opposition has taken court action to declare the president ineligible for office because his father was allegedly a foreigner.
Zambia's constitution says both parents of a presidential candidate must be citizens by birth or descent.
The Patriotic Front (PF) party alleges that President Banda's father was born in neighbouring Malawi.
Mr Banda's allies say he is a true Zambian and will contest upcoming elections.
The BBC's Mutuna Chanda in the capital, Lusaka, says the PF has filed its case in the High Court.
The party disputes Mr Banda's statement, made when he contested the 2008 poll, that his father was born in Chipata in eastern Zambia.
The PF insists that he was born in Nyasaland, which is known today as Malawi.
The ruling Movement for Multi-Party Democracy (MMD) party said it had no doubt that Mr Banda was eligible to contest elections, due on 20 September.
The PF had launched legal action because it feared being beaten at the polls, said the party's Deputy National Secretary Chembe Nyangu.
"In 2008, they could have blocked the president from standing but they did not. Why have they waited for three years?
"They know the MMD will win hands down," Mr Nyangu said.
Judgement in the case is due on Tuesday - a day before Mr Banda plans to file his nomination papers with electoral officials, our reporter says.
Former President Frederick Chiluba introduced the parentage clause in the constitution before the 1996 election.
It was widely believed to be a ploy to prevent his predecessor, Kenneth Kaunda, from running for office again.
Mr Kaunda's parents were said to be foreigners and the High Court declared the ex-president stateless.
He appealed against the ruling and the Supreme Court restored his citizenship.
Mr Chiluba was also accused of having foreign parents, an allegation he denied strongly.