Prince Philip has spent his 10th day in hospital, where he is expected to remain for at least the rest of the week.
Buckingham Palace has revealed the Duke of Edinburgh is being treated for an infection as his son Prince Edward said his father was "a lot better".
The palace said in a statement on Tuesday that Prince Philip is likely to remain in hospital "for several days", 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," said the short statement.
Prince Edward, Philip and the Queen's youngest child, told Sky News he had spoken to his father on the phone and the royal family were "keeping their fingers crossed".
The Earl of Wessex said the 99-year-old was "a lot better thank you very much indeed, and he's looking forward to getting out, which is the most positive thing. So we keep our fingers crossed."
Edward said the family had received "brilliant and lovely messages" and that "we really appreciate that and so does he".
Asked whether the duke was frustrated, Edward added: "Just a bit! I think that gets to all of us."
The duke has now been in hospital for seven nights as photos showed police guards waving in medical supplies at the entrance and UK media camped opposite.
Uniformed officers and a doorman in a top hat, all wearing face masks, stood watch in front of King Edward VII's Hospital in central London as Prince Philip entered his eighth day there.
Tired-looking camera crews camped opposite the door ready for any updates as concern remains high over the welfare of the 99-year-old, who was initially only expected to remain in hospital "for a few days of observation and rest", according to Buckingham Palace.
He was admitted to the private Marylebone hospital on Tuesday evening after feeling "unwell", but was driven the 26 miles (42km) from Windsor Castle and reportedly walked in.
The palace said he was in "good spirits" and had been taken to hospital as a "precautionary measure", but on Friday, royal sources indicated that the duke would probably need to remain in hospital into this week.
The sources said that Philip's doctor was "acting with an abundance of caution."
Visitors are banned from the hospital because of strict rules around Covid-19 and it was reported that the royals would only be allowed inside in "exceptional circumstances".
That's why fears grew when Prince Charles made the trip from Gloucestershire to visit his father in hospital on Saturday, entering through the rear entrance of the hospital, where he was met by security staff.
The Prince of Wales emerged looking sombre, however he then travelled back to his Highgrove home in a 200-mile (320km) round trip, rather than choosing to stay close to London, leading some to speculate that the duke's condition must be stable.
No other member of the royal family has yet been to see the duke in hospital. The Queen, 94, remains at Windsor, where she and Philip received at least one dose of the Oxford/AstraZeneca Covid-19 vaccine in January.
The palace did not initially reveal the reason behind Prince Philip's admission but sources said that it was not Covid-related. All patients at the hospital are tested for the virus before being admitted and in their rooms.
Prince William yesterday told reporters that his grandfather was "OK", giving a cheeky wink during a visit to a vaccination centre in eastern England. "Yes, he's OK, they're keeping an eye on him," the Duke of Cambridge, 38, told journalists.
The Sun reported that Prince Harry had called the Queen from California to ask about his grandfather's health, while reportedly quarantining in preparation to fly back to the UK by private jet. He is being kept "regularly informed" of Prince Philip's health, according to The Mirror.
Harry and Meghan Markle made their first appearance since announcing news they were expecting their second child on Monday, promoting their podcast Archewell Audio at Spotify's virtual Stream On event.
A pregnant Meghan was wearing a $4400 Oscar de la Renta dress as the pair spoke from their lounge at their Santa Barbara home.
It was also revealed at the weekend the pair had informed the Queen they would not be returning to the UK to re-assume royal duties, and would continue their life in the US where they are raising their one-year-old son, Archie.
Their 90-minute long TV interview with Oprah Winfrey for NBC was filmed this week and is due to air on March 7, with the Queen also due to deliver a TV address on the same evening for Commonwealth Day.
Philip, who turns 100 in June, has experienced a series of health scares in recent years.
The Queen's husband of 73 years was flown by helicopter to the same hospital from his Sandringham home in Norfolk just over a year ago for treatment for a "pre-existing condition". The Palace would not go into details about his ailment or the nature of his treatment at the time.
He had treatment for a blocked coronary artery in 2011, a bladder infection in 2012 and exploratory surgery on his abdomen in June 2013. He also spent two nights in hospital in 2017 just before his retirement from public life — a move the Palace insisted was not health-related.
In April 2018 he had a hip replacement operation but he had recovered well for Prince Harry and Meghan's wedding a month later.
At the start of 2019, the Duke was involved in a car crash when the Land Rover Freelander he was driving flipped over after a collision with another vehicle near the Sandringham estate.