The Duke of Edinburgh has been spending a second night in hospital after he was admitted as a precaution due to feeling unwell.
Prince Philip, 99, was said to be in “good spirits” after being taken to King Edward VII Hospital in central London on Tuesday evening.
His stay is not related to coronavirus and a palace source said the duke walked into the hospital unaided.
He is expected to be there for a few days of observation and rest.
The Queen, 94, has remained at Windsor Castle. Earlier on Wednesday, she spoke by telephone to the First Sea Lord, Admiral Tony Radakin, about the work of the Royal Navy at home and abroad.
Prince Charles and the Duchess of Cornwall also carried out official duties on Wednesday, visiting the Queen Elizabeth Hospital in Birmingham to speak to patients and NHS staff.
A No 10 spokesman said Prime Minister Boris Johnson sends his “best wishes” to the duke as he “undergoes a few days of rest in hospital”.
Last month, the palace announced that Prince Philip and the Queen had received Covid-19 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.
They celebrated Christmas quietly at their Berkshire residence, instead of the traditional family gathering at the Queen’s Sandringham estate in Norfolk.
In December 2019, Prince Philip – who turns 100 in June – was admitted to King Edward VII Hospital as a “precautionary measure” and received 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.
The duke’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 the Duke and Duchess of Cambridge’s children.