The British monarch, 95, has put off multiple trips and engagements recently, and the latest cancellation has deepened anxiety about her health.
Buckingham Palace announced the queen’s injury and withdrawal less than two hours before she was scheduled to appear at the ceremony, having signaled earlier on Sunday morning that she would be there.
The palace offered no additional details about the circumstances or severity of the queen’s back sprain, though an official said that she had not been hospitalized and hoped to continue carrying out “official light duties” in the coming week.
After being sidelined for almost a month — a rare absence in a nearly seven-decade reign — the queen had made it clear in recent days that she was determined to reappear in public at this ceremony.
The remembrance service, which honors Britain’s war dead, is one of the most solemn events on her calendar.
The ceremony went ahead on Sunday without the sovereign, but the pageantry took on even more solemnity than usual.
She spent a night at King Edward VII’s Hospital in London for what officials said were tests, before returning to Windsor Castle.
The queen looked alert and energetic at those events, though a photograph showed her using a walking stick during a ceremony at Westminster Abbey.
During her convalescence, the queen taped an address to the climate conference COP26 and conducted video calls with ambassadors.
Sandringham is where Philip retired after giving up his official duties and where the queen traditionally assembles the royal family to celebrate Christmas.
Among the questions this year is whether Prince Harry and his wife, Meghan, will be on hand.