Nelson Mandela has been admitted to hospital with a lung infection, the office of South African President Jacob Zuma says.
In a statement on its website, it said the former president had had a lung infection for several days.
His condition deteriorated on Saturday morning and he was transferred to a hospital in Pretoria. He is said to be in a “serious but stable condition”.
Mr Mandela, 94, has recently suffered a series of health problems.
His illness is described as a recurrence of a lung infection, which has troubled him repeatedly over recent years.
Mr Mandela, South Africa’s first black president, was taken to hospital at about 01:30 local time.
In April he was released from hospital after a 10-day stay caused by pneumonia.
Mac Maharaj, South Africa’s presidential spokesman, told the BBC he was receiving expert medical care.
Doctors were doing everything possible to make him comfortable and better, he added.