The Queen has postponed a meeting of her Privy Council advisers after being advised by her medical team to rest.
It comes a day after she was photographed for the first time in two months when appointing Conservative leader Liz Truss as Britain's new Prime Minister at Balmoral Castle in Scotland.
"After a full day yesterday, Her Majesty has this afternoon accepted doctors' advice to rest, a spokesperson for Buckingham Palace said.
"This means that the Privy Council meeting that had been due to take place this evening will be rearranged."
The Privy Council usually meets once a month and advises the Queen on the use of royal prerogative and statutory powers.
It is understood the Queen has remained at Balmoral, where she spends her summers, and has not visited hospital.
In a break from tradition Ms Truss, who was confirmed as new leader of the Conservative party on Monday, flew to visit the Queen at Balmoral, due to the 96-year-old monarch's ongoing mobility issues.
Ms Truss is the 15th prime minister of the Queen's reign.
The monarch has been suffering from what Buckingham Palace has called "episodic mobility problems" since the end of last year, reducing her public appearances in recent months, and missed last weekend's Highland Games; a usual highlight of her busy calendar.
