Princess Anne, sister of King Charles III, was in hospital on Monday after sustaining "minor injuries and concussion" following an incident at her country home Gatcombe Park, Buckingham Palace said.
"Her Royal Highness remains in Southmead Hospital, Bristol, as a precautionary measure for observation and is expected to make a full and swift recovery," said the statement, adding that the incident occurred late on Sunday.
"The king has been kept closely informed and joins the whole royal family in sending his fondest love and well-wishes to the princess for a speedy recovery," it added.
The 73-year-old royal will remain at the hospital in southwest England "unless or until her medical team advise otherwise".
The princess, a keen horsewoman who competed at the 1976 Montreal Olympics, has taken on extra engagements as the king and his daughter-in-law Catherine, Princess of Wales, both recover from cancer.
But her engagements for the week ahead will be postponed, said Buckingham Palace, including her planned attendance at Tuesday's state banquet for the emperor and empress of Japan.
The grounds of Gatcombe Park are renowned for hosting the Festival of British Eventing every year, which attracts the world's best equestrian stars.
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