Venezuela's President Hugo Chavez told the nation that his cancer had returned and named his successor should he not survive a new battle against the disease.
Mr Chavez, who won re-election on October 7, also said for the first time that if his health were to worsen, his successor would be vice president Nicolas Maduro.
"We should guarantee the advance of the Bolivarian Revolution," Mr Chavez said on television, seated at the presidential palace with Mr Maduro and other aides.
The president said tests had shown a return of "some malignant cells" in the same area where tumours were previously removed.
"I need to return to Havana tomorrow," he said, adding that he would undergo surgery in the coming days.
Mr Chavez called his third operation to remove cancerous tissue in about a year and a half a "new battle".
The 58-year-old outspoken left-winger first underwent surgery for an unspecified type of pelvic cancer in Cuba in June 2011 and had another surgery last February after a tumour appeared in the same area. He has also undergone chemotherapy and radiation treatments.
Mr Chavez said tests immediately after his re-election win had shown no sign of cancer. But he said he had swelling and pain, which he thought was due to "the effort of the campaign and the radiation therapy treatment".
"It's a very sensitive area, so we started to pay a lot of attention to that," he said, adding that he had reduced his public appearances.
Mr Chavez made his most recent trip to Cuba on the night of November 27, saying he would receive hyperbaric oxygen treatment. Such treatment is regularly used to help heal tissues damaged by radiation treatment.