TORONTO -- Prince Philip has been admitted to a hospital as a "precautionary measure," according to Buckingham Palace.

The BBC reported Friday that the 98-year-old Duke of Edinburgh travelled from Norfolk, England, to the King Edward VII Hospital in London for "observation and treatment in relation to a pre-existing condition."

The Palace confirmed in a statement to the broadcaster that he was taken "on the advice of His Royal Highness' Doctor."

He was not taken by ambulance as it was a planned visit, they added. A royal source told CNN that the Duke walked into the hospital where he is expected to remain for a few days.

On Friday, The Queen was seen taking a commuter train to Sandringham in Norfolk, where she will spend Christmas.

Philip, who retired from royal duties in 2017, has not been seen in public since the wedding of Lady Gabriella Windsor and Thomas Kingston at Windsor Castle near London in May.

CTV News royal commentator Richard Berthelsen called the news a “surprising development” as the Duke and Queen travelled in opposite directions this close to Christmas. The Queen’s decision to go to Sandringham the day after the opening of the U.K. Parliament sends a clear message, he said.

“She’s sort of showing a ‘carry on’ attitude. Probably the Duke himself is acutely aware that there will be huge public interest in this and wanted to signal that things were as normal as possible,” he said.

As for the Duke’s condition, there has never been official word on which specific pre-existing medical conditions the Duke may have.

“There have been some rumours about his health,” said Berthelsen, adding he has been in hospital before only on two occasions in the last decade or so. “That said, he was in incredibly good shape. He maintained sort of the same body type his whole life. He’s always been very fit and very active. That will stand him in good stead.”