"It's another lie and we are not going to talk about it. If anyone has to talk about Castro's illness it's Havana," the Cuban diplomat, who spoke on condition of anonymity, said.
"It's an invented story. I don't know anything about this."
Two medical sources, both working in a Madrid hospital with a surgeon who visited Castro in December, were cited in the El Pais article.
Peritonitis is a potentially fatal painful inflammation of the lining of the abdominal cavity often caused by infection in an organ. Unless it is successfully treated, it can lead to organ failure.
The hospital sources told the newspaper that Castro began suffering from an inflammation of his large intestine in the first half of last year, requiring an operation to remove part of the large intestine and rectum.
Continued infection impeded his recovery, requiring a second operation to remove all of the large intestine and rectum, the source added.
Castro was then hit with bile duct problems that have a "high rate of mortality" of around 80 percent, sources told El Pais
Last week, John Negroponte, the US national intellligence director, said that the Cuban president may have only days or months to live.
In a New Year's message issued on December 30, Castro told Cubans that he was recovering slowly from surgery and said his recovery was "far from being a lost battle."
Also in December, a Spanish doctor who examined Castro said he does not have cancer and could return to govern Cuba if he recovered fully from his surgery.