Madrid - Constant intermarriage between relatives was the main cause of the extinction of the Habsburg dynasty in Spain, which it ruled from 1516 to 1700, press reports Wednesday quoted a new study as saying.

Described as the first to apply genetics to a Spanish royal dynasty, the study confirms earlier hypotheses by many historians.

Spain's last Habsburg king, Chlares II, was unable to have children.

Researchers Gonzalo Alvarez Jurado, Francisco Ceballos and Celsa Quinteiro, who studied the blood relationships between 3,000 of the king's ancestors and relatives over 16 generations, attribute his infertility to constant inbreeding.

Charles also suffered from rickets - a disease which causes bones to soften - and stomach problems. He was physically short and weak, and aged prematurely before dying at age 39.

Spain's current Bourbon dynasty has experienced a much lesser degree of endogamy, reports quoted an initial analysis as indicating.

Marriages between relatives were common among European dynasties, which used them as a means to retain power.