President Jacob Zuma and ANC deputy president Cyril Ramaphosa visited Mandela last evening, Mac Maharaj, presidential spokesman, said in a statement.
The condition of the revered world leader, who was admitted to the Mediclinic Heart Hospital in Pretoria on June 8 has worsened in the past 24 hours.
"They were briefed by the medical team who informed them that the former president's condition had become critical over the past 24 hours," Maharaj said.
They also discussed his condition with Mandela's wife Graca Machel, who is believed to have been staying in an adjacent room at the hospital.
The latest statement from the Presidency is the strongest indication yet that all is not well with the ailing elder statesman, who will turn 95 in three weeks' time.
The news came amid a mounting public outcry after it was learnt that the ambulance transporting Mandela to the hospital from his home in Johannesburg in the early hours of June 8 broke down and paramedics had to treat him for almost forty minutes before a second one arrived.