MADRID, Dec 24 (Reuters) For more than 600 years, Spaniards have believed Prince Sancho de Castile's uncle poisoned him to become king of Spain, but studies of the boy's mummified body show the seven-year-old died of natural causes.

One of Spain's great royal legends may have been put to rest by medical tests that show Sancho, son of King Pedro I ''the Cruel'' of Castile, and a successor to the throne, was likely to have died in 1370 of a lung infection such as pneumonia.

Examinations of the prince's body have found no trace of cyanide, arsenic, mercury or any other poison his uncle, Enrique, was believed to have used to kill him, according to the convent where the prince's remains have lain since 1409.

''It appears the prince wasn't poisoned after all,'' the convent's Sister Maria Jesus Galan said yesterday.

The study led by the University of Granada and the pathology unit of Barcelona's Hospital Clinico found Sancho had inflamed lungs after chronic exposure to smoke, which was likely to have come from an open fire.

Enrique, the illegitimate son of Alfonso XI of Castile, killed his half brother Pedro I in the Castilian civil war in 1369 and became King Enrique I ''the Bastard'' of Castile.

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