Image source, ReutersImage caption, The Queen was pictured on Tuesday evening, hosting a Global Investment Summit at Windsor Castle
The Queen has cancelled a trip to Northern Ireland and has "reluctantly accepted medical advice to rest for the next few days", Buckingham Palace says.
The 95-year-old monarch will remain at Windsor Castle but is still expected to attend the COP26 climate change conference in Glasgow later this month.
The Queen is in "good spirits" but "disappointed" that the visit cannot go ahead, the palace said.
She was due to begin the two-day trip on Wednesday.
The nation's longest-reigning monarch has attended a series of events in recent days, hosting a Global Investment Summit at Windsor Castle on Tuesday evening.
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Earlier in the day, she held two audiences via video link, greeting the Japanese ambassador Hajime Hayashi and the EU ambassador Joao de Almeida.
On Monday, she held a virtual audience with the new governor-general of New Zealand, and at the weekend, she attended the races at Ascot.
It was revealed on Tuesday that the Queen had declined the Oldie of the Year award, from the magazine of the same name, saying: "You are only as old as you feel".
A Buckingham Palace spokesman said: "The Queen has reluctantly accepted medical advice to rest for the next few days.
"Her Majesty is in good spirits and is disappointed that she will no longer be able to visit Northern Ireland, where she had been due to undertake a series of engagements today and tomorrow.
"The Queen sends her warmest good wishes to the people of Northern Ireland and looks forward to visiting in the future."
Buckingham Palace is keen not to cause any alarm and has stressed that the Queen has "reluctantly accepted" the advice of doctors to rest for the next few days.
She has had a busy schedule of engagements over the past couple of weeks that would test the resilience of many people far younger than her.
I saw her last Tuesday at an event at Westminster Abbey.
It was the first time she had used a walking stick in public.
She also took a shorter route into the Abbey.
We were told this was "for her own comfort."
But she still looked incredibly well and engaged for a 95-year-old.
It is clear though that getting older takes its toll on us all and the Queen's diary will be carefully managed going forward.
The Queen had been due to arrive in Hillsborough in County Down on Wednesday afternoon and attend a church service marking the centenary of the formation of Northern Ireland in Armagh tomorrow.
An advance team was already in Northern Ireland making preparations for the two-day visit.
Sir Jeffrey Donaldson, leader of the Democratic Unionist Party, said on Twitter: "We thank Her Majesty for her good wishes to the people of Northern Ireland and trust that she will keep well and benefit from a period of rest.
"It is always a joy to have Her Majesty in Royal Hillsborough and we look forward to a further visit in the near future."
Church leaders in Northern Ireland said in a joint statement that they were sorry she would not attend the Service of Reconciliation and Hope in Armagh, and acknowledged "the significance of her commitment to the work of peace and reconciliation, which has meant a great deal to people throughout this island".
The Queen first travelled to Northern Ireland in 1945, just after the end of World War Two, when she was a princess. If it had gone ahead, this week's trip would have been her 26th visit.
Royal visits to Northern Ireland during its centenary year have included the first in line to the throne, Prince Charles who went to Belfast in May, and Prince William who visited Londonderry in September.