The careful wording of the palace statement suggests “something serious is possibly going on”, says former BBC royal correspondent Richard Sumner.
This comes following placed Queen Elizabeth II, the 96-year-old monarch under medical supervision, prompting concern from British political and religious leaders.
Britain’s longest-serving monarch has been dogged by health problems since last October that have left her struggling to walk and stand.
Speaking on BBC News, he says: “The whole policy of the palace has always been understatement and I’ve always suspected that possibly there was more than a mobility problem concerned here.”
Along with advancing age there may be other more complicated medical problems, he says.
“When you’re 96, of course you can’t expect to have the health of a 25-year-old.
“Hopefully this is a passing storm that can be overcome but I think that judging from the careful wording of the palace something serious is possibly going on here.”
Sumner adds: “People who’ve seen her say that mentally she’s sharp as a tack and very interested in everything still.”
He says it seems “her body is giving out on her” – though it’s not clear “quite in what way or how badly”.
All her children — heir to the throne Prince Charles, 73, Princess Anne, 72, Prince Andrew, 62, and Prince Edward, 58, were either at or heading to join her at the Balmoral royal estate in Scotland, aides said.
Also heading to Balmoral was Charles’s elder son Prince William along with his younger son Prince Harry and his wife, Meghan, who have been on a rare visit to Britain after abandoning royal life to move to the United States.