Doctors placed Queen Elizabeth II, who is 96, under medical supervision at Balmoral Castle, her estate in the Scottish Highlands.

Prince Charles traveled to Balmoral and will be at his mother’s side, together with his two brothers, sister and other members of the royal family, the BBC reported.

"The palace has gone further than they ever normally do," says the BBC's medical editor, Fergus Walsh.

He said the key phrase in the statement released by Buckingham Palace earlier, was "doctors were concerned".

He also highlighted the significance of family members arriving at Balmoral, where the Queen has been staying over the summer.

Until recently, her health has been "relatively robust" he explained. He described the monarch as a "permanent fixture", adding that "we've all lived our lives with the Queen".