King Charles III's sister Princess Anne is improving in hospital, her husband said Tuesday, after she suffered concussion in an incident at her country estate.

"She's doing fine -- slow but sure," Tim Laurence told reporters outside Southmead Hospital in Bristol, western England, where she was taken on Sunday evening.

Anne, 73, suffered minor injuries to her head and concussion at Gatcombe Park on Sunday evening and was airlifted to hospital.

It is believed she was kicked by a horse as she was walking within the protected perimeter of the sprawling estate, which hosts equestrian events.

Retired naval commander Laurence, her husband since 1992, said she was "recovering well", adding that they were both "profoundly grateful" to the medical teams that treated her at the scene and in hospital.

The Princess Royal, as she is also known, is a skilled horsewoman who competed at the 1976 Montreal Olympics, and has a reputation as the hardest-working royal.

She has stepped in to represent her brother while he postponed public engagements during his cancer treatment, and rode at his official birthday parade this month.

But the accident has forced her to withdraw from an appearance at a state banquet on Tuesday evening for visiting Japanese Emperor Naruhito, and cancel an upcoming trip to Canada.

Buckingham Palace said she was in hospital "as a precautionary measure for observation and is expected to make a full and swift recovery".