The Duke of Edinburgh is being treated for an infection and is not expected to leave hospital for several days, Buckingham Palace has said.
Prince Philip, 99, was admitted to the King Edward VII's Hospital in London last Tuesday as a precaution after feeling unwell, reports the BBC.
The reason for his admission was not initially given but the palace has now said he has an infection.
He is "comfortable and responding to treatment", a short statement said.
Both the duke and the Queen, 94, received Covid-19 vaccinations last month. Prince Philip's stay in hospital is not related to coronavirus.
The couple, who celebrated their 73rd wedding anniversary last November, have been spending lockdown at Windsor Castle with a small household staff.
A palace source previously said that the duke, who turns 100 in June, had walked into the hospital in Marylebone unaided.The Earl of Wessex said his father was "a lot better", telling Sky News: "He's looking forward to getting out, which is the most positive thing, so we keep our fingers crossed."
He said the family had received "brilliant and lovely messages" since Prince Philip went into hospital, adding: "We really appreciate that and so does he."
Prince Charles, 72, spent about 30 minutes at the hospital visiting his father on Saturday afternoon.
A Clarence House spokesman said he returned to his Highgrove residence after the visit.
Prince Philip has received treatment for various health conditions over the years, including a blocked coronary artery in 2011, a bladder infection in 2012 and exploratory surgery on his abdomen in June 2013.