Prince Philip is said to be "in good spirits" after being admitted to hospital in London last night.
He was hospitalised after feeling unwell and will be under observation for a few days, Buckingham Palace said in a statement.
The Duke of Edinburgh, 99, was taken to the private King Edward VII's Hospital in Marylebone as a "precautionary measure" advised by his doctor, according to the Daily Mail.
He was in "good spirits" and walked into hospital without help after being driven there by car, reports say.
The decision to hospitalise him came from "an abundance of caution". The Queen, 94, remained at Windsor Castle and is continuing with her royal duties, receiving a Royal Naval update from the First Sea Lord.
Prince Philip's illness is not related to Covid-19. It comes after it was announced on January 9 that he and the Queen had received their first doses of the vaccine, being high risk because of their old age.
The Palace has not confirmed whether they've received their second vaccine, which is meant to be given 12 weeks later to boost immunity, though some have had theirs sooner.
Buckingham Palace said in a statement: "His Royal Highness The Duke of Edinburgh was admitted to the King Edward VII Hospital in London, on Tuesday evening.
"The Duke's admission is a precautionary measure, on the advice of His Royal Highness's doctor, after feeling unwell. The duke is expected to remain in hospital for a few days of observation and rest."
The British prime minister Boris Johnson sent his best wishes to Philip for a quick recovery.
A spokesperson for the PM said, "The Prime Minister sends his best wishes to the Duke of Edinburgh as he undergoes a few days of rest in hospital."
This afternoon, police officers were seen guarding the private central London hospital where the duke is being treated.
He will celebrate his 100th birthday on June 10, around the same time that Prince Harry and Meghan Markle are expecting their second child.