Prince Philip is being treated for an infection in hospital, Buckingham Palace has confirmed, and is not expected to leave for several days.
The Duke of Edinburgh was taken to King Edward VII Hospital early last week as a "precautionary measure" after feeling unwell for several days.
On Tuesday, Buckingham Palace released a short statement saying the 99-year-old was "comfortable and responding to treatment".
"The Duke of Edinburgh remains at King Edward VII's Hospital where he is receiving medical attention for an infection.
"He is comfortable and responding to treatment but is not expected to leave hospital for several days."
The duke's youngest son, Prince Edward, told Britain's Sky News his father was improving and looking forward to leaving hospital.
"As far as I'm aware, he's a lot better… so he's looking forward to getting out, which is the most positive thing," he said.
Edward said his father was "just a bit" frustrated at being in hospital.
"You can only watch the clock so many times and the walls are only so interesting," he said.
"We've had some lovely messages from all sorts of people and we really appreciate that and so does he, I've been passing them on."
Speculation about the health of the duke had been mounting since his eldest son and heir to the throne Prince Charles travelled several hours by car to visit his father in hospital on Saturday.
Due to the COVID-19 pandemic in England, visits by family members can only occur in "exceptional circumstances", according to King Edward VII's hospital website.
On Monday, Prince William, the duke's grandson, told royal reporters his grandfather was doing "OK" during a public appearance.
"Yes, he's OK, they're keeping an eye on him," Prince William said on a visit to a vaccination centre in eastern England.
It is understood the duke was taken to hospital by car and walked into the hospital unaided, and his illness was not related to COVID-19.
He and the Queen have been staying at Windsor Castle, just outside of London, during the pandemic, and both received their COVID-19 vaccinations in early January.
Health battles in recent years
Prince Philip was previously admitted to the same hospital just before Christmas in 2019 for treatment for a pre-existing condition, and remained there for several days.
The prince, who turns 100 in June, retired from public life in August 2017 but occasionally makes appearances at royal engagements.
He spent Christmas 2011 in hospital after an operation to clear a blocked artery and he missed the end of celebrations to mark the Queen's 60th year on the throne in 2012 after being hospitalised with a bladder infection.
In 2013, the Greek-born former naval officer underwent "an exploratory operation following abdominal investigations".
He was admitted to hospital in 2017 for treatment for an infection, also arising from a pre-existing condition, and last year spent 10 days in hospital for hip replacement surgery.
In January 2019, Prince Philip was unhurt when his Land Rover flipped after a collision with another car near Sandringham.
He gave up his driving licence after police gave him a warning for driving without wearing a seatbelt.