ALASTAIR GRANT/POOL/AFP via Getty Queen Elizabeth last night
Queen Elizabeth has been advised by doctors to cancel a two-day visit to Northern Ireland that was set to begin today, Buckingham Palace said in a statement.
She has "reluctantly accepted medical advice" to stay at home to rest at Windsor Castle.
In a statement issued at around 11 a.m. on Wednesday, the palace said the monarch, 95, was in "good spirits." Her visit was to mark the 100th anniversary of Northern Ireland.
"The Queen has reluctantly accepted medical advice to rest for the next few days," the statement said. "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."
For more on Queen Elizabeth and other top stories, listen below to our daily podcast PEOPLE Every Day.
"The Queen sends her warmest good wishes to the people of Northern Ireland, and looks forward to visiting in the future."
ARTHUR EDWARDS/POOL/AFP via Getty Queen Elizabeth at the reception on Tuesday evening
The Queen's illness is not COVID-related, PEOPLE understands.
On Tuesday night, alongside Prince Charles, Prince William and other royals, she hosted international business and investment leaders at the castle for a Global Investment Summit reception. Guests included Bill Gates and U.S. Special Presidential Envoy for Climate John Kerry.
Jacob King-WPA Pool/Getty Images The Queen in Cardiff, Wales last week
She was at Ascot racecourse on Saturday and had opened the Welsh Parliament, or Senedd, last Thursday, in addition to carrying out a series of video calls and virtual audiences in the past week or two.
On Tuesday, it emerged that she had sent a witty response while politely turning down the chance to be deemed 'Oldie of the Year' by The Oldie magazine.
"Her Majesty believes you are as old as you feel, as such the Queen does not believe she meets the relevant criteria to be able to accept and hopes you find a more worthy recipient," the Queen's assistant private secretary Tom Laing-Baker said in a letter shared by The Oldie on Tuesday.