The Princess Royal has been admitted to hospital after suffering minor injuries and a head injury at her residence in Gloucestershire, Buckingham Palace has said.
Princess Anne, 73, is expected to make a full and speedy recovery following the incident at her Gatcombe Park estate, a statement said.
He remains admitted to Bristol's Southmead Hospital “for observation as a precautionary measure”.
“The King has been fully informed and joins the whole Royal Family in sending their heartfelt best wishes for the Princess's speedy recovery,” the statement said.
The cause of the injury is not yet clear, but it is believed to have been caused by a blow from the horse's head or legs.
The Princess Royal was expected to visit Canada this weekend.
Canadian media had reported she was scheduled to attend a ceremony at a war memorial in Newfoundland.
Princess Anne met veterans and their families at the Bayeux War Cemetery in France during the recent D-Day commemorations.
Princess Anne is considered one of the most hardworking royals, having completed numerous tasks.
Her sense of public duty and down-to-earth approach have made her one of the most popular royals, and her approval ratings in opinion polls have been consistently high.
She is known for her interest in horses and her achievements in equestrian sports, becoming the first member of the royal family to attend the Olympic Games in Montreal in 1976.
Princess Anne is the second child of Elizabeth II and Prince Philip and was third in the line of succession to the throne when she was born in 1950. She is now 17th in the line of succession.
She has been married twice. Her first husband, Captain Mark Phillips, is the father of her two children, Peter and Zara. Her second husband is Vice-Admiral Sir Timothy Laurence.