ZAMBIA’S main opposition party the Patriotic Front has accused President Rupiah Banda of lying about his parents' nationality and asked a court to block him from contesting next month's polls.

The country's constitution requires both parents of presidential aspirants to be born in Zambia, but in documents filed at High Court, PF secretary general Wynter Kabimba said Banda's father was a Malawian, and his mother Zimbabwean.

"We are claiming a declaratory judgment or order that the parents of President Banda are not citizens of Zambia by birth and that the MMD (Movement for Multi-party Democracy) cannot by law sponsor him as presidential candidate," Kabimba said in the court papers filed on Thursday.

Banda's spokesman Dickson Jere questioned why the opposition did not challenge Banda's candidature in 2008, when he won a special election following the death of former president Levy Mwanawasa.

Kabimba said that in the last election, Banda swore under oath that both his parents were born in Chipata, a town near the Malawi border. He said the PF now believes that Banda lied, and wants the court to rule on the matter.

Zambia’s former High Commissioner to Malawi, Milton Phiri, recently claimed Banda’s father — the late Bwezani Banda — was a Malawian immigrant labourer.

One of the lawyers representing President Banda, Eric Silwamba, said the PF’s civil proceedings against Mr Banda’s parentage were “incompetent”, insisting that it could not be heard by the High Court but the full bench of the Supreme Court.

Banda has set a general election for September 20, and is campaigning on the strong economic performance by Africa's biggest copper producer.

Former President Frederick Chiluba, who died last month, introduced the parentage clause in the constitution before the 1996 election.

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.