Princess Anne, the younger sister of King Charles III, was hospitalized on Sunday evening after suffering a concussion and other minor injuries in an unspecified accident, Buckingham Palace said on Monday.

The palace said Anne, 73, remained in Southmead Hospital in Bristol, but was expected to “make a full and swift recovery.” A spokesman said the princess “is recovering well, is in a comfortable condition and is being kept in hospital as a precautionary measure for further observation.”

The palace offered no details on the nature of the accident, which occurred while Anne was walking on the grounds of Gatcombe Park, a sprawling estate in Gloucestershire that serves as her country residence.

A person familiar with the royal family said there were horses in the area where Anne was injured, and that her head wounds were consistent with having had contact with the legs or head of a horse. Anne is an accomplished equestrian who competed for Britain in the 1976 Olympic Games in Montreal.

The palace said Anne’s injuries would force her to postpone a trip to Canada later in the week. She will also miss a state banquet for Emperor Naruhito of Japan, and his wife, Empress Masako, a palace official said, that is being hosted by King Charles and Queen Camilla on Tuesday evening.

Charles has been kept closely informed, the palace added, saying that he joined the whole royal family in “sending his fondest love and well wishes to the princess for a speedy recovery.”

Anne’s accident is the latest setback for Britain’s royal family, which has struggled with the illnesses of two of its most visible members, Charles and Catherine, the Princess of Wales, both of whom have been diagnosed with cancer.

Catherine, the wife of Prince William, made her first public appearance in several months on June 15 at a military parade commemorating the birthday of the king. Before her appearance, she said in a statement, “As anyone going through chemotherapy will know, there are good days and bad days.”

The king’s decision to go ahead with the state visit of the Japanese emperor has been viewed as a positive sign for his recovery. There have been news reports, not confirmed by Buckingham Palace, that Charles was also planning to go ahead with a visit in October to Australia and Samoa, which will host a meeting of the Commonwealth heads of government.

The sidelining of Anne will deprive the royal family of one of its most indomitable workers. She routinely logs more than 400 public events a year — often the most of any family member — and with the thinning of the ranks of senior royals in recent years, Anne has shouldered an even heavier load.

With the deaths of Queen Elizabeth II and her husband, Prince Philip, the illnesses of Charles and Catherine, and the departure of Prince Harry and his wife, Meghan, for the United States, the royal family has had a much smaller pool of members to pick up its busy calendar of royal duties and obligations.

After the queen’s death in September 2022, Anne, her only daughter, assumed unofficial status as the senior female figure in the House of Windsor, though in royal protocol she is outranked by Camilla. Princess Charlotte, the 9-year-old daughter of Catherine and William, is the most senior female royal in terms of succession.

Like her brother Charles, Anne has weathered ups and downs in her personal life, as well as in her public image. As a young woman, she was sometimes viewed as haughty and sharp-tongued. But she became respected for her philanthropic work, serving as president of Save the Children from 1970 to 2017.

Anne’s first marriage to a fellow equestrian, Mark Phillips, ended in divorce, but she has had a stable second marriage to Timothy Laurence, a retired Royal Navy officer who served as an aide to the queen. An official said Mr. Laurence accompanied Anne to the hospital. He and her two children, Zara Tindall and Peter Phillips, were on the estate at the time of her accident, the official said.

In her public appearances, Anne is known for her brisk and unflappable demeanor — a characteristic she has displayed in other challenging episodes. In 1974, she survived a kidnapping attempt, telling the gunman who ordered her out of her car to hold her for ransom: “Not bloody likely!”