The Duke of Edinburgh has been spending a third night in hospital after being admitted on his doctor’s advice.
Prince Philip, 99, was earlier said to be in “good spirits” after being taken to King Edward VII Hospital in central London as a precaution on Tuesday.
His stay is not related to coronavirus and is expected to last for a few days.
The Queen, 94, has remained at Windsor Castle and performed her first face-to-face event of the year on Thursday when she knighted a royal aide.
Vice-Admiral Sir Tony Johnstone-Burt was made Knight Commander of the Royal Victorian Order during a private socially-distanced ceremony at Windsor.
Sir Tony, a former Royal Navy officer, runs a department that handles official and private entertaining and hospitality across all royal residences.
The Duke of Cambridge carried out several virtual conversations with front-line NHS staff on Thursday, while the Princess Royal held a virtual meeting in her role as patron of the Scottish Rugby Union.
The Prince of Wales and the Duchess of Cornwall have also continued official duties.
In a pre-recorded video message for a virtual meeting hosted by the British Asian Trust, Prince Charles said on Thursday it was a “tragedy” not everyone was benefitting from the “extraordinary achievement” of the coronavirus vaccine.
After feeling unwell, Prince Philip was advised to go into hospital by his doctor – though the exact reason for the admission is undisclosed.
A palace source said the duke had walked into the hospital in Marylebone unaided.
Last month, the palace announced that Prince Philip and the Queen had received Covid vaccinations from a household doctor at Windsor Castle.
The couple have been spending the recent lockdown in England at Windsor Castle with a small household staff, nicknamed HMS Bubble.
The Queen and Prince Philip celebrated Christmas quietly at their Berkshire residence, instead of the traditional family gathering at the Queen’s Sandringham estate in Norfolk.
In December 2019, the duke, who turns 100 in June, was admitted to the same hospital – King Edward VII Hospital – as a “precautionary measure” for treatment for a “pre-existing condition”.
He was discharged on Christmas Eve after four nights and driven to Sandringham, where he spent Christmas with the Queen.
He has received treatment for various other 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.
Prince Philip’s most-recent official engagement saw him hand over his role of colonel-in-chief of The Rifles in a ceremony at Windsor to the Duchess of Cornwall, who received the role separately.
Last November, the Queen and Prince Philip marked their 73rd wedding anniversary by releasing a photograph of themselves opening a card made for them by their great grand children, Prince George, Princess Charlotte and Prince Louis.