The Queen has postponed her Privy Council meeting and has been advised by doctors to rest, Buckingham Palace says.
A palace spokesman said: “After a full day yesterday, Her Majesty has this afternoon accepted doctors’ advice to rest.
“This means that the Privy Council meeting that had been due to take place this evening will be rearranged.”
The Queen remains at Balmoral Castle, where she appointed Liz Truss as the new prime minister on Tuesday.
The latest advice does not involve a hospital stay for the 96-year-old monarch, who has ongoing mobility issues.
She had been due to hold the Privy Council virtually on Wednesday.
During the proceedings, Ms Truss would have taken her oath as First Lord of the Treasury and new cabinet ministers would have been sworn into their roles, and also made privy counsellors if not already appointed as one in the past.
There is no constitutional issue with the delay to the proceedings, the palace said.
During Tuesday’s audience, Her Majesty was pictured smiling, dressed in a blouse, cardigan and a Balmoral tartan skirt, and using a walking stick.
She first met outgoing leader Boris Johnson before appointing the new prime minister, Ms Truss, at Balmoral for the first time in her reign, in a break from tradition.
She would normally appoint prime ministers from Buckingham Palace or Windsor Castle.
However, the monarch was advised to remain at her Balmoral residence in Aberdeenshire, where she has been enjoying her summer holiday.
It was thought to be best for the Queen not to travel back to London due to her intermittent mobility issues.
She is not due to return until early October.
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Since last autumn, Her Majesty has suffered with mobility issues and now regularly uses a walking stick.
This year, she has been forced to cut several official engagements short, and notably missed much of her Platinum Jubilee celebrations, only appearing briefly on the palace balcony.
In June, she missed Royal Ascot for the first time since her coronation, with the Duke of Kent taking her place in leading the royal carriage procession.
Earlier in May, the Queen missed the State Opening of Parliament for the first time in 59 years on doctors’ orders after experiencing “episodic mobility issues”.
And she withdrew from the traditional Easter Sunday service at St George’s Chapel in Windsor this year.
The Queen also contracted COVID-19 back in February, and later spoke about how it left her feeling “very tired and exhausted”.