Queen Elizabeth's husband, Prince Philip, was admitted to hospital on Tuesday evening as a precautionary measure, Buckingham Palace said.
"The Duke's admission is a precautionary measure, on the advice of His Royal Highness's Doctor, after feeling unwell. The Duke is expected to remain in hospital for a few days of observation and rest," the statement said.
The prince, who is aged 99, was admitted to hospital for a short period at the end of 2019.
He stepped down from official engagements in August 2017.
