STORY: The queen, Britain's longest-reigning sovereign and the world's oldest monarch, has been suffering from what Buckingham Palace has called "episodic mobility problems" since the end of last year.
"Following further evaluation this morning, The Queen’s doctors are concerned for Her Majesty’s health and have recommended she remain under medical supervision," the palace said in a statement.
"The queen remains comfortable and at Balmoral."
Her eldest son and heir Prince Charles and his wife Camilla have traveled to her Scottish home Balmoral Castle, where she is staying, along with his eldest son Prince William, officials said.
Last October, Elizabeth spent a night in hospital and she has been forced to cut back on her public engagements since then. On Wednesday she canceled a virtual meeting with senior ministers after being advised to rest by her doctors.
The previous day she had been pictured appointing Liz Truss as the country's new prime minister at Balmoral.
A palace source said immediate family members had been informed and played down speculation that the monarch had suffered a fall.
Elizabeth has been queen of Britain and more than a dozen other countries since 1952, and earlier this year celebrated her 70th year on the throne.