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 past.
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The Queen appointed the new prime minister 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 was 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.
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Royal correspondent
It is telling that the palace has felt it has had to tell us once again that the Queen has been advised to rest by her doctors.
I think, in essence, that’s because alarm bells may have started ringing, people would have started questions, if that Privy Council meeting was not in the court circular.
Certainly over the last 12 months, there has been a change in terms of what the Queen has been able to carry out.
There is definitely now more of this common-sense approach within the palace and within the family.
We all need to get used to the fact there are some things she is just not going to be able to do.
Five years ago it was difficult to talk about transition, it was difficult to talk about a change or a handing over – now we just have to get used to it.
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”.
