In a statement, the 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's doctor.”
While some reports state that Philip walked into the hospital, the Daily Mail reports that the Duke was flown by helicopter from Norfolk to London, fuelling concerns over his health.
According to the publication, the prince has been "battling a flu-like illness for weeks" and suffered a fall before being rushed to hospital.
Speaking about the Duke’s health in recent months, one source said: “The Duke has had a bad turn recently and has been looking much frailer of late.
“He's a remarkable man who has always prided himself on how robust he is, but he is six months off his 99th birthday and age takes its toll.
“He hasn't been looking terribly well of late.”
The Duke of Edinburgh retired from a life of royal duties in 2017, and since then the Queen's husbands health has been a serious cause for concern.
And while the Palace has downplayed the Duke’s admission as ‘pre-planned’, a royal expert claimed the Queen’s reaction suggests that it’s business as usual for the monarch.
Royal commentator Richard Fitzwilliams previously told Express: “The fact that the Queen went by train as usual to Sandringham for Christmas today, indicated that although Prince Philip had to go to hospital for observation and treatment of a pre-existing condition, it was business as usual as much as it could be.”
But while the Queen’s actions may support the claim Philip's admission was routine, Richard said the major concern was if he were to need surgery at the of age 98.
“From the facts available, it seems that the only natural cause for concern is that at the age of 98 any such procedure must be worrying,” he said.