major health scare.
However, the government insisted the anti-apartheid hero was having a "routine test" and there was "no need for panic".
A spokesman for President Jacob Zuma said that doctors were treating 94-year-old Mandela for a pre-existing condition.
He was in hospital for 18 days in December with a lung infection and gallstones before being discharged on Boxing Day.
Mr Mandela is revered for being a leader of the struggle against racist white rule in South Africa and for preaching reconciliation once he emerged from prison in 1990 after 27 years behind bars.
He served one five-year term as president before retiring from public life.
His health has been deteriorating in recent years.
