The Queen has pulled out of a service at St Paul's Cathedral because she is suffering from a cold and feels "under the weather".
The 92-year-old had been due to attend a service marking the 200th anniversary of the Order of Michael and St George.
It is understood no doctor has been called and that the Queen will still travel from Buckingham Palace to Windsor this afternoon for the weekend as planned.
Prior to Thursday, the Queen has attended a number of occasions in the last few days, giving no indication that she felt ill.
Elizabeth II was last seen in public at the Young Leaders' awards ceremony - a celebration of inspirational young people from across the Commonwealth - at Buckingham Palace where she was joined by the Duke and Duchess of Sussex.
While on Sunday she attended a polo match with the Duke of Edinburgh. and was regularly seen at Royal Ascot last week.
A spokesperson for the Queen confirmed she was "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."
Angelina Jolie is one of the guests expected to attend the service as a recipient of an honour from the Order, which is granted by the Queen on the advice of the Prime Minister.
It is for distinguished services in connection with foreign affairs and service overseas.
The Queen is well-known for her strong constitution and no-fuss approach to her infrequent illnesses.
Last month she had eye surgery to remove a cataract.
She was treated as a day patient and did not cancel any engagements nor appearances, but was spotted wearing sunglasses.
In 2016 - the year she turned 90 - the monarch called time on her overseas travels, leaving long-haul destinations to the younger members of her family.
But she still has a busy diary of events, and in 2017 carried out 296 engagements.