Buckingham Palace said Princess Anne suffered “minor injuries and concussion” in the incident on Sunday night.
It was understood emergency services were called to the scene of the accident, which took place in Bristol’s Gatcombe Park grounds on Sunday, to treat injuries possibly caused by a collision with a horse.
The 73-year-old princess was then taken to Southmead Hospital for treatment and is expected to remain there until the end of the week.
In a statement released on Monday, the palace said: “Following an incident at Gatcombe Park last night, The Princess Royal suffered minor injuries and concussion.
“Her Royal Highness remains in Southmead Hospital in Bristol for observation as a precaution and is expected to make a full and speedy recovery.”
“The King has been fully informed and, together with all the Royal Family, sends her his deepest love and wishes her a speedy recovery.”
Princess Anne, a working member of the royal family, is the King’s younger brother.
The palace said the princess was “recovering well.”
Doctors have advised the princess to postpone her plans for next week, including a trip to Canada later this week.
The princess will also be unable to attend tomorrow’s state dinner hosted by her brother in honour of Emperor Naruhito of Japan, who is visiting the UK this week.
“His Royal Highness apologises to anyone who may experience inconvenience or disappointment as a result,” a palace source said.