Balcells, who never attended university, died on Sunday in Barcelona, a spokeswoman for her agency told AFP without giving further details.
"The family wants privacy. They will issue a statement on Tuesday," she added.
Balcells is credited with being one of the forces behind the so-called "Latin American Boom", an unprecedented burst of creativity, genre-bending works and success that came out of Latin America in the 1960's and 1970's.
She went on to represent other big names such as Argentina's Jorge Luis Borges, Chile's Pablo Neruda and Isabel Allende, and Uruguay's Juan Carlos Onetti.
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"I can't imagine my life without Garcia Marquez," she said during a television interview in 2012.
"All of a sudden I found myself surrounded by geniuses," she added, referring to the writers she represented, many of them who were living in Barcelona at the time.
After finding success with Latin American writers, she turned to Spanish authors and represented Camilo Jose Cela, among others.
In 2014 her agency merged with one owned by US agent Andrew Wylie, creating Balcells & Wylie, one of the biggest international literary agencies.
Spanish Prime Minister Mariano Rajoy hailed Balcells as an "indispensable figure" in Spanish literature in a Twitter message after her death was announced.