Anderson Mazoka believed he won the 2001 elections

Mr Mazoka, 63, lost narrowly to President Levy Mwanawasa in 2001 presidential elections and claimed the polls had been rigged.

Party officials said he died after prolonged kidney complications and had spent a lot of time receiving medical treatment since his 2001 defeat.

The UPND leader was tipped as a strong challenger in polls later this year.

Mr Mazoka was a wealthy businessman and the former managing director of South Africa's mining giant, Anglo American Corporation, in Zambia. He also was general manager of Zambia's railways.

He had strong support in the south of the country, where the UPND won all the parliamentary seats in 2001.

The UPND had formed an alliance with two other opposition parties, so his death now leaves another prominent politician, Edith Nawakwi, well placed to give President Mwanawasa a close contest, reports suggest.