A statement from the office of current president Jacob Zuma said: “During the past few days, former President Nelson Mandela has had a recurrence of lung infection.
“This morning at about 1.30am, his condition deteriorated, and he was transferred to a Pretoria hospital. He remains in a serious but stable condition.
“The former President is receiving expert medical care, and doctors are doing everything possible to make him better and comfortable.
It is the second time in recent months Mr Mandela has been in hospital and the fourth health scare in six months for the anti-apartheid hero. Mr Mandela has a history of lung problems dating from when he contracted tuberculosis as a political prisoner.
The Nobel Peace Prize laureate, who was imprisoned for 27 years by the racist white regime, became the first democratically elected president of South Africa in 1994.
He spent 27 years on Robben Island and in other jails for his attempts to overthrow the white-minority government.
Now Mandela is seen as a global symbol of tolerance and the struggle for equality.