epa09992097 People gather along the Mall to watch a fly past form the Red Arrows during Queen Elizabeth II's Platinum Jubilee celebrations in London, Britain, 02 June 2022. Britain is enjoying a four day holiday weekend to celebrate Queen Elizabeth II's Platinum Jubilee marking the 70th anniversary of her accession to the throne on 06 February 1952. EPA-EFE/ANDY RAIN 
The queen had been due to attend the service at St Paul's Cathedral on the second day of celebrations for her Platinum Jubilee.
Earlier on Thursday she appeared on the balcony of Buckingham Palace, waving to thousands of well wishers.
"The Queen greatly enjoyed today’s Birthday Parade and Flypast but did experience some discomfort," the palace said.
"Taking into account the journey and activity required to participate in tomorrow’s National Service of Thanksgiving at St Paul’s Cathedral, Her Majesty with great reluctance has concluded that she will not attend."
(Reporting by Kate Holton; Editing by Cynthia Osterman)