Prince Philip, Queen Elizabeth's 99-year-old husband, was admitted to hospital on Tuesday evening as a precautionary measure with an ailment that is not COVID-related.
The prince, whose official title is the Duke of Edinburgh, walked into hospital unaided after feeling unwell for a short period, a royal source said.
"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.
He is at the King Edward VII Hospital in London where he is expected to remain for a few days of observation and rest.
He does not have a COVID-19 related illness, the 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 earlier in January.
Philip is rarely seen in public after he stepped down from official engagements in August 2017. He was previously admitted to hospital at the end of 2019.
The Greek-born former naval officer has had several stays in hospital over the last decade, including treatment for an infection in 2017 and hip replacement surgery.