LONDON - Britain's Prince Philip, the duke of Edinburgh, has been admitted to hospital as a "precautionary measure," Buckingham Palace said in a statement Friday. The 98-year-old duke, who is married to Queen Elizabeth II, will undergo treatment for a preexisting condition on his doctor's advice, the palace said.

Philip made the 115-mile journey from his residence in Sandringham, Norfolk, to London's King Edward VII Hospital on Friday afternoon and is expected to be kept there for observation over the next few days. The duke is not believed to have been transported to the hospital by ambulance and entered the hospital on foot.

"I'm told that Philip walked into hospital - there was no emergency admittance - and will probably stay in for a couple of days," the Daily Mail's royal correspondent Rebecca English tweeted Friday.

As the duke arrived in the capital, the queen was photographed braving the rain in a powder-pink ensemble as she boarded a train from London's Kings Cross station back to the royal Sandringham Estate in Norfolk, where she will spend the Christmas break. It has been a busy week for the monarch, who formally opened Britain's Parliament on Thursday and hosted her annual Christmas lunch.

While many members of the royal family attended the lunch, Philip did not appear to be present at the family festivities. The duke and duchess of Sussex, Meghan and Harry, were also absent and are believed to be vacationing at an undisclosed location with their son, Archie Harrison.

The duke, who married the queen in November 1947, has not been seen in public much in recent months and underwent a hip operation in April 2018. He officially retired and stepped down from royal duties in May 2017.

"Well, I can't stand up much!" the duke said at the time when a guest at a reception the queen was hosting told him she was sorry to learn that he was standing down. He is thought to have undertaken 22,000 solo engagements during his time supporting the queen in her duties around the world.