Princess Anne has sustained "minor injuries and a concussion" after an incident on Sunday evening, Buckingham Palace has said.
It is understood emergency services were called to the scene of the incident on the Gatcombe Park estate, Gloucestershire, on Sunday, to treat injuries believed to have been caused by impact with a horse's head or leg.
The princess, 73, was then taken to Bristol's Southmead Hospital for treatment, where the Olympic-medal winning horsewoman is expected to stay until later this week.
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"The Princess Royal has sustained minor injuries and concussion following an incident on the Gatcombe Park estate yesterday evening," the palace said in a statement on Monday.
"Her Royal Highness remains in Southmead Hospital, Bristol, as a precautionary measure for observation and is expected to make a full and swift recovery.
"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."
Princess Anne, the younger sister of the King, is a senior working royal who lives on the Gatcombe Park estate.
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The palace said the princess is "recovering well".
Doctors have recommended she postpone engagements for the week ahead. It is understood that includes a trip to Canada scheduled for the end of this week.
"Her Royal Highness sends her apologies to any who may be inconvenienced or disappointed as a result," a palace official said.
The no-nonsense royal's life passion is horses
Rhiannon Mills
Royal correspondent
What exactly happened isn't entirely clear but from the few details it seems this was a freak accident.
Like all members of the royal family, Princess Anne was brought up around horses but for her it's been a life passion, she even competed in the Olympics.
Her skills as an accomplished horse rider were on display just over a week ago as she took part in the Trooping the Colour procession on horseback alongside Prince William and Prince Edward.
At 73, it was remarkable to see her trying to keep in check her horse Noble, who at times appeared to have other ideas about where he wanted to go.
So for her to sustain head injuries consistent with being kicked by a horse or headbutted is a surprise, but as anyone who loves horses will tell you, these things happen.
Over the years, Princess Anne has affectionately gained a reputation as being a very no-nonsense member of the royal family.
She seems to tolerate the limelight when she has to and like her father Prince Philip, she doesn’t like a fuss.
But as an important senior working royal, it was clearly decided by the palace that an announcement about her health should be made, especially as she remains in hospital.
She's already sent an apology to those she was meant to see on engagements this week and I understand it's also put a stop to a trip to Canada on behalf of the King.
Concussion for anyone is always treated with the utmost caution, but as the palace keeps stressing -she remains under the attention of medics purely as a precaution.
The princess will also be unable to attend tomorrow's state banquet organised by the King in honour of Japanese Emperor Naruhito, who will be visiting the UK alongside his wife, the Empress Masako.
Prime Minister Rishi Sunak said on X he wished the princess a "swift recovery" and said everyone in the country is "immensely fond" of her.
Labour leader Keir Starmer said he was wishing her a "speedy recovery".
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The Royal Family has been hit by a series of illnesses in the past year, with both the King and the Princess of Wales undergoing treatment for undisclosed forms of cancer.
Kate said earlier this month she was "not out of the woods yet" and has "a few more months" of cancer treatment to go, as she gave an update on her health.
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She made her first public appearance since her diagnosis announcement at Trooping the Colour, to celebrate the King's official birthday.
