Prince Philip has been hospitalised just days before Christmas in a dash from Sandringham to London for treatment.
The Duke of Edinburgh travelled from Norfolk to the King Edward VII Hospital in London for observation and treatment in relation to a pre-existing condition.
The 98-year-old is expected to be in hospital for a few days.
He was not taken by ambulance and it was a planned admission – with reports he entered the hospital walking and checked himself in.
In a statement, Buckingham Palace said: "The Duke of Edinburgh travelled from Norfolk this morning to the King Edward VII Hospital in London for observation and treatment in relation to a pre-existing condition.
"The admission is a precautionary measure, on the advice of His Royal Highness' Doctor."
Philip has suffered a number of ailments over the years, including being treated for a blocked coronary artery at Papworth Hospital in Cambridgeshire in December 2011.
He was also treated for bladder infection in June 2012 forcing him to miss the Queen's Diamond Jubilee celebrations.
Exploratory surgery on his abdomen followed in June 2013 and he began to use hearing aids by 2014, aged 93.
He later pulled out of a Battle of Jutland anniversary event in June 2016 citing a minor ailment which was followed soon after by his retirement from public duty in 2017.
Philip retired from his Royal duties in August 2017, but has seen periodically since in public.
His son Prince Charles has taken up some of his duties, including at the State Opening of Parliament, as he prepares to be the next monarch.
The Duke was also involved in a car crash in January this year while driving near the Sandringham estate.
He was left hanging upside down after his Landrover Freelander flipped and had to undergo a medical following the crash.
Buckingham Palace announced the duke voluntarily surrendered his driving licence "after careful consideration" following the crash.
He also wrote to the other driver involved in the prang, wishing her a "speedy recovery".
The duke made his first public appearance since the car crash when attending the Order of Merit lunch alongside the Queen at Windsor Castle in May.
Philip was previously hospitalised in April, 2018, for a hip operation and in June, 2017, for treatment for an infection.
It comes amid reports Philip had to step into the Royal controversy over his son Prince Andrew's handling of his friendship with Jeffrey Epstein.
The Duke of Edinburgh is understood to have had “strong words” with his son hours before a Panorama interview with former Epstein sex slave Virginia Robert Guiffre.
Andrew categorically denies any allegations against him.
The 59-year-old is believed to have been forced to drive 140 miles to see his father and Prince Charles.
Charles is understood to have spoken to his younger brother to reinforce the Queen’s wish Andrew step back from royal duties.
The stern chat came a week after Prince Philip summoned Andrew for a face to face meeting in Norfolk at Sandringham Estate.
“It was a tense meeting and Philip told his son he had to take his punishment," an insider.
"“There was no screaming or shouting but Philip told him in no uncertain terms that he had to step down for the sake of the monarchy.
“Philip doesn’t like trial by the media but he is realistic enough to realise that Andrew’s actions were a danger to the very fabric of the royal family.
“It was agreed they would have another meeting with Charles."