Britain's Queen Elizabeth II has cancelled a trip to Northern Ireland and “reluctantly accepted medical advice to rest for the next few days”, Buckingham Palace said on Wednesday.
The palace said there was no Covid-related illness behind the medical advice.
The queen, 95, was at Windsor Castle for a public engagement on Tuesday.
Last week, she was seen in public using a walking stick for the first time when not recuperating, when she attended a service marking the centenary of the Royal British Legion.
She is now resting at Windsor Castle but is still expected to attend events linked to the Cop26 climate change conference in Glasgow at the end of the month, while her Platinum Jubilee will be marked next year.
“The queen has reluctantly accepted medical advice to rest for the next few days,” a Buckingham Palace spokesman said on Wednesday.
“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 hosted a Global Investment Summit at Windsor Castle on Tuesday evening.
Among those at the reception were 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 present and the royals mingled with guests in the castle's green drawing room and St George's Hall.
The queen, who is due to celebrate 70 years on the throne next year, still keeps a busy diary of events and audiences, and deals with her daily red boxes of official papers.
She 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.
Last weekend, she enjoyed a day at the races at Ascot, and on Monday held a virtual audience with the new governor general of New Zealand.
On Tuesday, it was revealed the queen turned down the Oldie of the Year trophy because she felt she does not meet the criteria, believing "you are only as old as you feel".
In the next few weeks, she is set to travel to Glasgow for the Cop26 climate change conference, attend Remembrance Sunday at the Cenotaph in London and record her Christmas Day message.
The royal household is preparing for the queen's Platinum Jubilee, with a weekend of festivities being held next summer.
Over four days in June, she will attend Trooping the Colour event, a service of thanksgiving at St Paul's Cathedral, a visit to the Epsom Derby, a live concert at Buckingham Palace and a Jubilee pageant.
Northern Ireland politicians sent best wishes to Queen Elizabeth.
"Wishing Her Majesty The Queen all the very best as she takes a few days' rest. I look forward to meeting her in Northern Ireland in the future," Northern Ireland Secretary Brandon Lewis tweeted.
Sir Jeffrey Donaldson, leader of Northern Ireland's Democratic Unionist Party, tweeted his best wishes for the queen as she begins her rest.
"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."
Prince Philip, the queen's husband of 73 years, died in April, aged 99.