Vilma Espin Guillois, the wife of Cuba's interim leader has died, aged 77. State-run Cuban television said she passed away on Monday from complications following a long undisclosed illness.

She was a pioneering Cuban feminist who was among the original leaders of the movement that brought her husband Raul Castro's brother, Fidel, to power 47 years ago.

Vilma Espin Guillois was Cuba's de facto first lady for decades as Cuban leader Fidel Castro was divorced. She was said to be ill for the past year.

The American-educated daughter of a wealthy industrial magnate from eastern Cuba, some biographers say she turned her back on her upbringing, having trained as a chemical engineer at the Massachusetts Institute of Technology. She met the Castros as a young urban rebel who battled against Fulgencio Batista's dictatorship in the 1950s.

She married the younger Castro in 1959 after Batista fled Cuba and the triumphant revolutionaries marched into Havana. Some called it a marriage made in revolution.

Although rumours circulated for years that Espin and Raul Castro had separated, they were sometimes seen together, with their four children, and there was never any official word of divorce.

Espin had been less visible over the past three years amid rumours of her worsening health. She was notably absent in March during celebrations of International Women's Day – an important date on the island.

The Cuban government has declared a day of official mourning in Espin's honour beginning on Monday evening, with national flags being lowered to half mast.