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.
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.”
The Queen’s decision is understood to be unrelated to coronavirus.