LONDON (AP) _ South African leader Nelson Mandela has endorsed a longtime aide in the struggle against apartheid to take over as president when he steps down in 1999.

Mandela, who makes a state visit to Britain this week, has long been known to favor his affable, pipe-smoking deputy Thabo Mbeki as his successor, but has not said so directly until Sunday.

Mbeki ``is very talented and very popular and if the (African National Congress party) elected him I would feel that they had made the right choice,'' Mandela told the British Broadcasting Corp.

A skilled negotiator, Mbeki has been given greater responsibilities in the past 18 months as Mandela's workload has been cut to protect the president's sometimes fragile health.

Mandela, who turns 78 this year, said he still intends to stand down in three years.

`What is clear is that a man of 81 cannot really lead a robust country such as ours. Our young democracy would require a comparatively young leadership,'' he said.

But Mandela said it was ``not the culture'' within his ruling ANC for the president to choose his successor. He also said after retirement he plans to spend time with his grandchildren and do a bit of farming.

Mbeki, the son of Mandela's fellow Robben Island prison inmate Govan Mbeki, holds a master's degree in economics from Sussex University in England. During decades of exile in Britain, Zambia and elsewhere, he served the ANC as tactician, fund-raiser and envoy.

He was a member of ANC teams that negotiated South Africa's transition to all-race democracy. Mandela named him his first deputy after South Africa's first multiracial elections in 1994.

He has been criticized, however, for failing to quickly resolve some of South Africa's problems, and his love of high living has earned him the reputation of an elitist.

His strongest challenger for the presidency is former union boss and lawmaker Cyril Ramaphosa, who announced recently he was leaving Parliament to take a position in private business.

During his four-day visit to Britain, Mandela will address a joint session of the House of Commons and the House of Lords, receive eight honorary degrees and visit charity projects in south London.