"This morning at about 1.30am his condition deteriorated and he was transferred to a Pretoria hospital," spokesperson Mac Maharaj said in a statement.
"He remains in a serious but stable condition."
Maharaj said doctors were doing everything they could to make Mandela "better and comfortable".
"President Jacob Zuma, on behalf of government and the nation, wishes Madiba a speedy recovery and requests the media and the public to respect the privacy of Madiba and his family," Maharaj said.
On April 6, Mandela was discharged from hospital after spending nine days receiving treatment for recurring lung problems. The Nobel Peace Prize laureate has suffered lung ailments before.
Early in March, he was admitted to a Pretoria hospital for a scheduled check-up and was discharged the following day.
In December last year, Mandela underwent an operation to remove gallstones and treat the recurring lung infection. He was discharged after an 18-day stay and placed under home-based high care at his Houghton home.
In January, the presidency said Mandela had made a full recovery from the surgery and continued to improve. In February last year he was admitted to hospital for a stomach ailment.
At the time, the presidency said Mandela underwent a diagnostic procedure to investigate the cause of a long-standing abdominal complaint.
In January 2011, Mandela was taken to Milpark Hospital for routine tests relating to respiratory problems.
Mandela's last major public appearance was in July 2010, at the final of the Fifa World Cup at Soccer City stadium in Johannesburg.
Since then he had spent his time between Johannesburg and his ancestral village of Qunu in the Eastern Cape.
The African National Congress and its alliance partners wished former president Nelson Mandela a speedy recovery after he was admitted to hospital on Saturday.
"We send to him our well wishes for a speedy recovery so that he may soon be discharged to return to the care and comfort of his home," ANC spokesman Jackson Mthembu said in a statement.
The Congress of SA Trade Unions said it members were thinking of Mandela at this time.
"Our 2.2 million members, thousands of working people, millions of South Africans and countless more freedom loving people around the world are thinking of you today and wishing that you make a full recovery and soon return home," the trade union federation said in a statement.
The SA Communist Party sent its support to Mandela's family.
"To the family and to the former president, you are in our thoughts… the SACP appeals to all that the former president and the family be given the necessary privacy at this time," the SACP said.
Mthembu said: "The ANC hopes and trusts that president Mandela and his family will be afforded the necessary respect and privacy while he is in hospital, we have no doubt that Tata will continue to receive expert medical care and attention at this time." – Sapa