Buckingham Palace said Queen Elizabeth II is under medical supervision as doctors are "careful of Her Majesty's health".
Thursday's announcement comes the day after the 96-year-old monarch called a meeting of his private council and asked for calm.
The palace said the Queen was "at ease" and was at Balmoral Castle in Scotland
"The news from Buckingham Palace at lunchtime will concern the whole nation."
are with the Queen and her family at this time," he wrote on Twitter.