The Queen has missed the Remembrance Sunday service at the Cenotaph in London as she has sprained her back.
Buckingham Palace said the monarch, 95, was "disappointed" not to attend the event.
The palace previously said it was the Queen's "firm intention" to attend the service, after taking time away from her duties for health reasons.
She made the decision to miss the event "with great regret" this morning, a statement said.
As in previous years, a wreath was laid on her behalf by the Prince of Wales.
The Duchess of Cornwall, the Duke and Duchess of Cambridge, the Earl and Countess of Wessex and The Princess Royal also still attended as planned.
At his Downing Street news conference on the COP climate deal, Prime Minister Boris Johnson said he had an audience with Her Majesty last week and "she is very well".
This is a blow. A blow to those gathered here on Whitehall on a grey November morning, a blow to the many who had hoped to see the Queen after a fortnight of rest, and without doubt a blow to the Queen herself - Remembrance Sunday is when she, who has devoted her life to service, pays her tribute to those who served.
It is, as one palace official puts it, "incredibly unfortunate timing". When the palace announced that the Queen would cancel her official engagements for two weeks it went out of its way to say that it was her "firm intention" to attend the National Service of Remembrance.
It's understood that the Queen will not need hospital treatment. It looks as if a car ride from Windsor and a period of standing in the cold watching the ceremony were just impossible given her back sprain.
In previous years, she may have weathered the pain. But there is no getting round the fact that she is 95 years old. For a period, at least, quieter times beckon.
The Queen's back sprain is unrelated to her doctor's recent advice to rest, news agency PA Media reported.
Doctors had advised the monarch to rest until mid-November after she spent a night in hospital on 20 October for checks - her first overnight hospital stay in eight years.
However, she did undertake some light duties during that time, including meeting ambassadors via video link from Windsor Castle.
She also recorded a video message for the COP26 climate summit in Glasgow, after pulling out of attending the event in person.
And she missed the Festival of Remembrance at London's Royal Albert Hall on Saturday, which was attended by Prince Charles, Camilla, Prince William and Catherine.
The Remembrance Sunday service would have been her first duty in public after her hospital stay last month.
Buckingham Palace said in a statement: "The Queen, having sprained her back, has decided this morning with great regret that she will not be able to attend today's Remembrance Sunday service at the Cenotaph.
"Her Majesty is disappointed that she will miss the service."
The Queen will be continuing light duties at Windsor.
The monarch, who lived through World War Two as a teenager, is head of the armed forces.
She has only missed six other Cenotaph ceremonies during her reign - on four occasions when she was on overseas visits and in 1959 and 1963, when she was pregnant with her two youngest children.
The Queen had maintained a typically busy schedule in October before being admitted to hospital, but was recently seen using a walking stick at a Westminster Abbey service, the first time she has done so at a major event.
On Tuesday, she returned to Windsor Castle following a long-planned weekend away at her Sandringham home in Norfolk.
Last week, she was spotted driving her car near Windsor Castle, in an area where she is known to take her Corgi dogs out for walks.