The Duke of Kent, the Queen's cousin, has suffered a mild stroke.
Buckingham Palace said the 77-year-old royal was taken to an undisclosed London hospital on Monday, where he was assessed by doctors.
A 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".
"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 in hospital.
It is understood that the Queen, who herself has been ill recently with the symptoms of gastroenteritis, has been informed.
The Duke is President of the Stroke Association and its Chief Executive Jon Barrick wished him well, saying: "Our thoughts are with him and his family and we wish him a very speedy recovery.
"The Duke of Kent has been President of the Stroke Association since our inception over 20 years ago and he has shown tremendous support and dedication to our cause."
The Duke inherited his royal title at the age of six after his father's life was tragically cut short in a plane crash in 1942.
After a lengthy military career, he has spent many years performing royal duties on behalf of his cousin the Queen.
He is perhaps best known for being President of the All England Lawn Tennis and Croquet Club and presenting the Wimbledon trophies each summer.
The royal is also the country's top Freemason - the secret society's Grand Master of the United Grand Lodge of England.
"Eddie" Kent was a respected Army officer who also acted as Britain's international trade ambassador for 25 years.
His wife the Duchess of Kent has withdrawn from public life, dropping her HRH title, after suffering from depression and other illnesses.
She now works as a music teacher, preferring to be known as plain Katharine Kent.
The couple have three children George, the Earl of St Andrews, Lady Helen Taylor, and Lord Nicholas Windsor.
