The Queen Elizabeth II 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 world’s longest-reigning monarch has attended a series of events in recent days, hosting a Global Investment Summit at Windsor Castle on Tuesday evening.
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 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.