The Duke of Edinburgh was on Thursday in a London hospital as a precautionary measure after being admitted on Tuesday.
Prince Philip, 99, the husband of Queen Elizabeth II, was said to be in “good spirits” in the private King Edward VII Hospital in Marylebone.
He was admitted on the advice of doctors after feeling unwell. He walked into the hospital unaided and does not have Covid-19.
Buckingham Palace said on Wednesday that the Duke was expected to remain in hospital for a few days of “observation and rest”. Police officers were pictured outside the hospital on Thursday.
UK Prime Minister Boris Johnson's office said that he had sent his "best wishes to the Duke of Edinburgh as he undergoes a few days of rest in hospital".
The Queen, 94, is at Windsor Castle, where the couple have self-isolated for most of the pandemic. They were inoculated against Covid-19 last month.
The royal family continued its duties on Wednesday, with the Queen speaking to Royal Navy Admiral Tony Radakin from Windsor, while Prince Charles and the Duchess of Cornwall visited a hospital in Birmingham, central England.
Prince Philip retired from public duties in 2017 at the age of 96 following two nights in hospital with an infection. He had a hip replacement operation in 2018.
In January 2019, he was uninjured after his vehicle was involved in a traffic accident near the royal Sandringham estate in eastern England. Two people were injured in the collision.
He spent four nights at King Edward VII Hospital in December 2019, where he was treated for what was described as a "pre-existing condition".
Since his retirement, he has rarely been seen in public. He attended the wedding ceremony of his granddaughter Princess Beatrice in July 2020, shortly before celebrating his 73rd wedding anniversary.
The latest spell in hospital renewed concerns about the Duke's health before his 100th birthday in June this year.
The Duke, the longest-serving consort in British history, was born on the Greek island of Corfu. His uncle, King Constantine of Greece, was forced to abdicate and Philip fled the country with his parents and four sisters, settling in France.
Philip was introduced to Princess Elizabeth, the future queen, in July 1939 and they kept in touch during the Second World War. The pair married at Westminster Abbey in London in 1947.
The couple celebrated their platinum wedding anniversary in November 2017 at Windsor. The Queen has referred to her husband as her "strength and stay".
