The Princess Royal was hospitalized with minor head injuries and concussion, possibly caused by a horse.
Buckingham Palace said Princess Anne, 73, is expected to make a full and speedy recovery from the accident at her Gatcombe Park estate in Gloucestershire on Sunday.
Sources said her injuries appeared to be caused by impact with the horse’s head or leg.
“The King is fully aware of the situation and, together with all the Royal Family, offers her his heartfelt wishes and much love for her speedy recovery,” the palace statement added.
Princess Anne’s visit to Canada this weekend has been postponed, and she will not attend a state dinner for a visit to Japan on Tuesday.
The Princess Royal was injured on Sunday evening while walking on her grounds and nearby horses.
The cause of the head wound has not been confirmed, but the lack of certainty reflects the possibility that the princess may have suffered a concussion in the incident.
Emergency services were called to the residence and the princess was taken to Southmead Hospital in Bristol.
A Buckingham Palace statement said the princess was conscious and would remain in hospital as a “precautionary measure for observation” until later this week.
Southmead Hospital is an NHS hospital that is part of North Bristol NHS Trust and has several key specialities including neuroscience.
Her husband, Admiral Sir Tim Lawrence, her daughter Zara and son Peter were on the property at the time, and he later went to the hospital with her.
“The Princess Royal suffered minor injuries and concussion following an incident in the Gatcombe Park grounds last night,” Buckingham Palace said in a statement.
According to the NHS, a concussion can be the result of “temporary damage to the brain caused by a bump, blow or shock”.
It can cause forgetfulness, fatigue and difficulty concentrating, and it can take weeks for you to fully recover.
Princess Anne’s hospitalization is the latest health crisis facing the royal family.
King Charles and the Princess of Wales were both diagnosed with cancer this year and have had their royal duties limited.
Known as one of the most hardworking royals, Princess Anne continued to help with many official duties in their absence, helping to bolster the dwindling royal ranks.
She had a series of events planned this week, including an official visit to Japan, with other events scheduled in Scotland, Hampshire and Gloucestershire.
Her sense of public duty, combined with her practical attitude, has earned her consistently high approval ratings in opinion polls and established her as one of the most popular royals.
She is known for her love of horses and her achievements in equestrian sports, becoming the first member of the royal family to compete in the Olympic Games at the 1976 Montreal Games.
The second child of Queen Elizabeth II and Prince Philip, she was third in line to the throne at birth and is now 17th in line.
She has been married twice: to her first husband, Captain Mark Phillips, with whom she has two children, Peter and Zara.