The Queen has cancelled a trip to Northern Ireland and has “reluctantly accepted medical advice to rest for the next few days”, Buckingham Palace said.
The 95-year-old monarch is said to be in good spirits but disappointed not to be able to carry out the two-day trip, which was due to begin today.
The Queen has had a busy programme and hosted a major Global Investment summit at Windsor Castle on Tuesday evening, where she looked bright and cheerful as she carried out her royal duties.
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.”
The Queen’s decision is understood not to be related to coronavirus.
Buckingham Palace would not comment on whether the head of state had received her booster Covid-19 jab, but given her age it is likely she has already had it.
The nation’s longest reigning monarch is resting at Windsor Castle.
She is still expected at this stage to attend events linked to the Cop26 climate change conference in Glasgow at the end of the month.
On Tuesday, it was revealed the Queen had turned down the Oldie of the Year trophy because she feels she does not meet the criteria, believing “you are only as old as you feel”.
The Queen, who is just five years away from her 100th birthday, is due to celebrate her Platinum Jubilee – 70 years on the throne – next year.
She still keeps a busy diary of events and audiences and deals with her daily red boxes of official papers.
The Queen held two audiences on Tuesday via video link from Windsor Castle, greeting the Japanese ambassador Hajime Hayashi and the EU ambassador Joao de Almeida, followed by the investment summit in the evening.
Among those at the Windsor reception for billionaire business leaders, presidential envoys and tech entrepreneurs were Prime Minister Boris Johnson, Microsoft co-founder Bill Gates, US climate envoy John Kerry, Poppy Gustafsson, chief executive of cyber security firm Darktrace, and Hamed Bin Zayed Al Nahyan, managing director of Abu Dhabi Investment Authority.
The Prince of Wales and the Duke of Cambridge were also present and the royals mingled with guests in the castle’s green drawing room and St George’s Hall.
Neither the royal family nor the guests were wearing masks.
On Wednesday, England’s chief medical officer Professor Chris Whitty urged people to get vaccinated against Covid-19 or take up the offer of a booster shot.
He also stressed the importance of wearing masks in crowded indoor spaces after the NHS Confederation called for the introduction of “Plan B” measures which would make face coverings compulsory in some settings in England.
At the weekend, the Queen enjoyed a day at the races at Ascot, and on Monday held a virtual audience with the new governor-general of New Zealand.
Last week, she travelled to Cardiff to deliver a speech at the ceremonial opening of the sixth Welsh Senedd in Cardiff.
The Queen’s husband of 73 years the Duke of Edinburgh died in April at the age of 99.
The monarch has been pictured out and about at the Royal Windsor Horse Show and Ascot in the months since, and still enjoys riding her own ponies.
Last week, she used a walking stick for what is believed to be the first time at a major public event when she attended a service marking the centenary of the Royal British Legion.
