"Mrs Mandela went to the hospital for routine medical checkup on Wednesday, March 8. A decision was taken to admit her for further observation," the statement said.

Former first lady Madikizela-Mandela, 80, was in hospital for several weeks last year for back surgery.

During her ex-husband's 27-year incarceration for his fight against apartheid, Madikizela-Mandela campaigned for his release and for the rights of black South Africans, undergoing detention, banishment and arrest.

Mandela died in 2013 and made no mention of Madikizela-Mandela in his will. She plans to challenge the will, according to media reports.