The Queen pulled out of a service at St Paul’s Cathedral on Thursday because she is feeling “under the weather”.
The 92-year-old was due to attend the service in the capital to mark the 200th anniversary of the Order of St Michael and St George.
She was instead be represented by the Duke of Kent, Prince Edward, who is the Queen’s cousin.
The shock announcement yesterday left royal fans fearing for the Queen’s health.
A spokeswoman for Buckingham Palace said: “The Queen is feeling under the weather today and has decided not to attend this morning’s service at St Paul’s Cathedral marking the 200th anniversary of the Order of Michael and St George.
“Her Majesty will be represented by the Duke of Kent as the grand master of the order.”
However, the news comes after the Queen underwent eye surgery to remove a cataract in May.
She underwent the delicate procedure at the private King Edward VII’s hospital in London, where she was treated as a day patient.
The planned operation resulted in a success.
It is understood no doctor has been called and the Queen still travelled from Buckingham Palace to Windsor on Thursday afternoon for the weekend as planned.
Her programme of events in Scotland next week is also still due to take place.
The cancellation yesterday follows a busy month for the Queen, who enjoyed five days at her favourite week of the month, Royal Ascot.
She also joined grandson Prince Harry and his wife Meghan Markle this week at her Young Leaders Awards at Buckingham Palace.
The UK’s longest-serving monarch is known for her fit bout of health, unlike husband Philip who has been in and out of hospital this year.
Her son Prince Andrew once described his mother as being incredibly fit for her age.
The 92-year-old still rides her Fell ponies at Windsor and is often spotted behind the wheel, enjoying a drive around her estates.
She also still carries out hundreds of royal events each year, thanks to her good health.
In 2017, attended 296 events – although she no longer takes part in royal visits abroad, leaving that to her children and grandchildren.