Friends of Nelson Mandela, jailed leader of the banned African National Congress, believe he was moved from Robben Island, the bleak, windswept prison where he has been held for the past 17 years, because his captors felt he had become too influential among the 400 or so political prisoners there.

Mandela, who is serving a no-parole life sentence for plotting the overthrow of South Africa's white-minority government, had been organizing a large-scale educational program for his fellow prisoners.

His influence on the island reportedly had grown considerably during the past year in other ways, extending even to some of the prison warders, although their political views differ fundamentally from his.

Mandela was moved secretly from Robben Island, about 40 miles offshore from Cape Town, to a new jail in the city of Pollsmoor on April 1. News of the move only leaked out last week.

Three other congress leaders serving life sentences were moved with him: Walter Sisulu, Raymond Mhlaba and Andrew Mlangeni.

Prison officials have confirmed the move, saying only that it was for "administrative reasons." They added that Mandela was not moved for health reasons.

Mandela will be more comfortable physically in the mainland jail, although the separation from his fellow dissidents remaining on the island is a blow. Robben Island is a low-lying, damp, windswept place often lashed by heavy seas.

Some of the prisoners on Robben Island are young Soweto students who were jailed after the 1976 uprisings in that Johannesburg township. They will be released in a few years, and the authorities may have feared the influence the outspoken Mandela had on them.

Mandela's educational program began with about 40 prisoners, but he recently sent messages to friends to raise funds for nearly all the prisoners on the island.