The Queen won't be attending a thanksgiving service at St Paul's Cathedral on Friday (local time) after experiencing "some discomfort" during Platinum Jubilee events.
Buckingham Palace said the Queen, with "great reluctance", is not attending due to the journey and activity required to take part.
It is not immediately known how the news will affect Jubilee events over the weekend.
The Queen has had trouble moving around in recent months, and has pulled out of many public events.
READ MORE: Queen's Platinum Jubilee kicks off with pomp
However, the palace said she is looking forward to participating in Thursday night's Beacon lighting event at Windsor Castle.
The Queen drew wild cheers from the tens of thousands gathered to celebrate her 70 years on the throne, when she stepped onto the Buckingham Palace balcony on Thursday.
Her fans sported Union Jack flags, party hats or plastic tiaras as they watched the Trooping the Colour, a military parade that has marked each sovereign's official birthday since 1760.
It was an explosion of joy in a massive crowd, one of the first big gatherings since the Covid-19 pandemic began more than two years ago.
Elizabeth, who became queen at 25, is Britain's longest-reigning monarch and the first to reach the milestone of seven decades on the throne.
The jubilee is being commemorated with a four-day holiday extravaganza and events including a concert at Buckingham Palace on Saturday (local time) and a pageant staged by thousands of performers drawn from schools and community groups around the country on Sunday (local time). Thousands of street parties are planned nationwide, repeating a tradition that began with the Queen's coronation in 1953.
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