Castro Diaz-Balart, also known as "Fidelito" because of how much he looked like his father, had initially been hospitalised for depression and then continued treatment as an outpatient.

"Castro Diaz-Balart, who had been attended by a group of doctors for several months due to a state of profound depression, committed suicide this morning," Cubadebate website said on Thursday.

Fidelito was born in 1949 out of his father's brief marriage to Mirta Diaz-Balart before he went on to topple a US-backed dictator and build a communist-run state on the doorstep of the United States during the Cold War.

Through his mother, he was the cousin of some of Castro's most bitter enemies in the Cuban American exile community, US Representative Mario Diaz-Balart and former US congressman Lincoln Diaz-Balart.

A nuclear physicist who studied in the former Soviet Union, Castro Diaz-Balart had been working as a scientific counselor to the Cuban Council of State and Vice-president of the Cuban Academy of Sciences at the time of his death.

Previously, from 1980 to 1992, he was head of Cuba's national nuclear programme, and spearheaded the development of a nuclear plant on the Caribbean's largest island until his father fired him.

Cuba halted its plant plans that same year because of a lack of funding after the collapse of Cuba's trade and aid ties with the ex-Soviet bloc and Castro Diaz-Balart largely disappeared from public view, appearing at the occasional scientific conference.

His death came just over a year after that of his father on November 26, 2016, aged 90.