Prince Philip, Queen Elizabeth's 99-year-old husband, was admitted to hospital on Tuesday as a precautionary measure after feeling ill with an ailment that is not related to COVID-19.
The prince, whose official title is the Duke of Edinburgh, walked unaided into the private King Edward VII Hospital in London after feeling unwell for a short period, a royal source said.
The queen, 94, remains at Windsor Castle near London, where the pair have been staying during Britain's coronavirus pandemic. Both received their first dose of the COVID-19 vaccine in January.
"The Duke's admission is a precautionary measure, on the advice of His Royal Highness's doctor, after feeling unwell," Buckingham Palace said in a statement.
"The Duke is expected to remain in hospital for a few days of observation and rest."
A royal source said Philip did not have a COVID-19-related illness and that it was not an emergency.
Within an hour of his arrival, camera crews and photographers from Britain, Australia, France and Germany had gathered outside the hospital where policemen stood guard.
"Really sorry to hear that His Royal Highness, the Duke of Edinburgh is in hospital," British Health Secretary Matt Hancock said. "Sending him my best wishes for a speedy recovery."
A former naval officer renowned for his sometimes brusque manner and humour, Philip married Elizabeth in 1947, five years before she became queen. He is now by far the longest-serving consort of any British monarch.
The couple's marriage, the earlier-than-expected inheritance of the British throne, and family life with four children has been charted in recent years by the Netflix drama The Crown.
Philip is now rarely seen in public. He stepped down from official engagements in August 2017 after completing more than 22,000 solo events and thousands more alongside the Queen.
During the pandemic the royal family has visited hospitals and frontline workers, either online or in person, to thank them for their efforts. Prince Charles, the heir to the throne, visited staff in a Birmingham hospital earlier on Wednesday.
The royal family has also been in the news in recent days after Philip's grandson Prince Harry and his American wife Meghan announced they were expecting their second child, and that they had agreed to a sit-down interview with Oprah Winfrey.
Among the topics they will discuss are why the couple quit the royal family to spend more time in America last year.
The Greek-born Philip has had several stays in hospital over the last decade, including treatment for an infection in 2017 and hip replacement surgery. He was previously admitted to hospital at the end of 2019.
The Duke is due to turn 100 in June. The tabloid newspapers have reported that he does not want any public fuss.