The Duke of Kent, the Queen's cousin, has suffered a mild stroke.
The 77-year-old royal was taken to an undisclosed London hospital in the early hours of Monday where he was assessed by doctors.
A Buckingham Palace spokesman said: "He was taken ill in the early hours of Monday and admitted to hospital for assessment."
He added that medical staff "diagnosed him as having had a mild stroke".
The Buckingham Palace spokesman added: "He's still being assessed and it's not thought to be serious.
"My understanding is he's feeling well and looking forward to resuming official engagements as soon as possible."
It is thought the Duke, whose first name is Edward, is still being treated at a London hospital.
It is understood the Queen, who herself has been ill recently with the symptoms of gastroenteritis, has been informed.