London (CNN) Prince Philip has been admitted to hospital over a "pre-existing" condition, Buckingham Palace said in a statement on Friday. A royal source told CNN he was not taken there in an ambulance, but walked in.

"The Duke of Edinburgh traveled from Norfolk this morning to the King Edward VII Hospital in London for observation and treatment in relation to a pre-existing condition," the statement said.

Philip retired from public life back in 2017. The 98-year-old patriarch is rarely seen publicly these days and is believed to spend most of his time at Wood Farm on the Sandringham estate.

The statement added that the admission was a "precautionary measure, on the advice of His Royal Highness' Doctor."

The announcement from the palace came shortly after Queen Elizabeth II made her way to Sandringham in Norfolk. The 93-year-old caught a train from London's King's Cross train station on Friday.

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