Mwanawasa has consolidated his grip on the MMD

After 10 years in power, Mr Chiluba tried to change the constitution to allow him to stand for a third term as president in last year's disputed elections.

After months of arguments and uncertainty, Mr Chiluba did not stand in the poll, which the opposition say was rigged in favour of Mr Mwanawasa.

Mr Mwaanga denied that Mr Chiluba's resignation was the result of a split in Zambia's ruling party, but the BBC's Bivan Saluseki in Lusaka says that there have indeed been arguments between Mr Chiluba and Mr Mwanawasa.

Mr Mwaanga said that Mr Mwanawasa did not want to become MMD leader, but our correspondent says that the president has now consolidated his control over the party.

Retirement

Earlier this month, a judge ordered Mr Chiluba to stop using government-owned facilities and personnel.

He had to give up his Mercedes, a number of other vehicles, the keys to a government-owned house in the fashionable district of Kabulonga and both security guards and domestic staff.

The ruling was based on one of Mr Chiluba's own laws - that past presidents are not entitled to any state benefits if they stay involved in active politics.

The legislation was specifically targeted at his predecessor, Dr Kenneth Kaunda.

The French news agency, AFP, reports that Mr Chiluba would be eligible for $67,500 worth of retirement benefits if he quits his party post.