The Queen will not attend a service at St Paul's Cathedral because she is feeling "under the weather".
Buckingham Palace said the monarch would not be going to the ceremony marking the 200th anniversary of the Order of St Michael and St George.
Heath warnings have been issued over the continuing heatwave although it is unclear if the cancellation is weather-related.
With temperatures set to soar to highs of 33C on Thursday, Public Health England (PHE) said it was “vitally important” that people look out for vulnerable friends, families and neighbours.
The palace confirmed The Duke of Kent, Prince Edward, will attend the service in the place of the 92-year-old royal.
The Duke of Kent is Grand Master of the Order.
It is believed the Queen will still go to Windsor this afternoon for the weekend and her engagements in Scotland will continue as normal next week.
The Queen, who is Britain’s longest-reigning monarch, has not called a doctor.
She has been reducing her workload in recent years and Prince Philip officially retired from royal duties last year.
