Prince Philip was flown by helicopter from Sandringham to London after suffering serious 'flu-like' symptoms, according to reports.
And the Duke of Edinburgh had previously been left bed-bound following a fall before he was admitted with a "pre-existing condition", it is understood.
Sources said he has struggled to eat and has been looking increasingly frail in recent weeks.
The 98-year-old is being treated at the King Edward VII Hospital, and Buckingham Palace has said it was a planned admission.
He walked into the hospital, having been driven to West London after arriving in the capital by helicopter, it is understood.
An insider 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."
The source also told the Daily Mail : "He hasn't been looking terribly well of late."
The duke has lived alone on a farmhouse in Wood Farm, Sandringham, since he retired from public life in August 2017.
A palace statement said after his admission yesterday morning: "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."
Prince Philip is expected to be in hospital for "a few days", according to The Sun .
A source close to the Duke said he has stopped eating and didn't want to go out after a fall.
They stated: "The Duke had a bad fall and was confined to bed for a couple of days.
"He didn’t break anything, but it gave everyone a scare. He suffers from gout which makes him more irritable and he lost his appetite."
None of the royals appear to be in a rush to get to his bedside, suggesting his condition is not thought to be overly serious.
The Queen is in Norfolk, while Prince Charles is believed to be in Highgrove, Gloucestershire.
Prince Harry and Meghan Markle have no plans to come back from their holiday in Canada, it is claimed.
As his age has advanced beyond 90, Philip has faced a number of scares and had a hip replacement operation last year.
Abdominal surgery, bladder infections and a blocked coronary artery have seen him admitted to hospital on a number of occasions too.
His latest admission to London's King Edward VII's Hospital on Friday morning was pre-planned and is for observation and treatment for a pre-existing but undisclosed condition.
