President Edgar Lungu can stand in presidential elections due in 2021, Zambia's top court ruled on Friday, finding that he would not be breaching a constitutional two-term limit.

The ruling will strengthen Lungu's hand at a time when Zambia, Africa's second largest copper producer, is grappling with mounting debt and an economic slump caused by depressed commodity prices.

Lungu's first period in office lasted only one year and six months, when he took over after the death of the previous elected president Michael Sata. He then won election in a disputed vote to a second, full term in August 2016.

Lungu's supporters argued that the constitution says a president is only deemed to have served a term if he is in office for at least three years of the full five-year term.

Constitutional Court President Hildah Chibomba said the ruling by the seven judges of the court had been unanimous.

"Our answer is that the (Lungu's first) presidential tenure of office...cannot be considered as a full term," Chibomba said.

The official opposition party was not available for comment.

Lungu told supporters in November that Zambian judges could plunge the southern African country into chaos if they made any "adventurous" rulings, according to the Zambia Daily Mail and Times of Zambia.