The El Pais website quoted medical sources at the Gregorio Maranon hospital in Madrid, from where a doctor traveled to Havana last month to examine the 80-year-old revolutionary. Castro has not been seen in public since being taken ill in July.

The communist government in Havana treats the leader's health like a state secret and the hospital would not comment on the new report.

But there has been mounting speculation in recent weeks over the condition of Castro. Last week, the office of John Negroponte, the outgoing director of US intelligence, said Castro was "terminally ill" and may have only days to live.

El Pais said the Cuban leader is suffering from a "serious infection of the large intestine following three failed operations and various complications that have degenerated into peritonitis."

Peritonitis is a potentially fatal inflammation of the lining of the abdominal cavity. It is often caused by an infection of an organ and often leads to organ failure unless treated.