Katy Skerrett, senior coroner for Gloucestershire, reached a conclusion of accidental death and said Dame Jilly, 88, died as a result of a traumatic subdural haematoma.
Her novels often portrayed the scandals, sex lives and social circles of the wealthy horse-loving country set.
Rivals recently became a hugely successful TV series starring A-listers such as David Tennant and Danny Dyer.
When her death was announced by her children, Queen Camilla described the author as “a legend”.
