Sicilian banker Michele Sindona, who has been in a deep coma since yesterday morning, ingested a massive dose of cyanide in his jail cell two days after being sentenced to life imprisonment for planning the 1979 murder of a Milan bank examiner, doctors confirmed today.

Judicial authorities in Milan were trying to determine whether Sindona, who in 1980 gave himself an overdose of digitalis from which he almost died, had attempted suicide or whether someone had tried to murder him.

After a series of tests, doctors at the hospital in Voghera, outside Milan, said that substantial traces of cyanide in his blood, urine and gastric fluids indicated that Sindona, 65, had swallowed a potentially lethal dose of poison, probably in his morning milk or coffee.

Italian Justice Minister Mino Martinazzoli told an angry Chamber of Deputies this morning that contrary to Sindona's habit of eating breakfast at the table in his cell at Voghera's maximum security prison, he drank his coffee yesterday in his bathroom, out of the sight of his guards, and had emerged saying, "I've been poisoned." Shortly afterward, he collapsed and was hospitalized.

Later the Italian news agency ANSA quoted investigators as suggesting that the poison had been in milk, citing testimony by guards that Sindona had moaned after emptying his glass.

Members of Parliament who took the floor demanded to know how it was possible that cyanide could be introduced into a maximum security prison and given to a prisoner who, like Sindona, was under 24-hour guard with three television monitors recording his every move, even when he was in his private bathroom.

Martinazzoli, a Christian Democrat, met with Socialist Prime Minister Bettino Craxi to discuss the situation and said he would resign if official negligence was demonstrated. He said that since September 1984, when Sindona was extradited from the United States, where he was serving a 25-year sentence for bank fraud, he had been given maximum protection and that all his food had been brought to his cell in sealed metal containers.