The prime minister and members of the Royal Family have led the nation in honouring the country's war dead on Remembrance Sunday, as the Queen missed the service at the Cenotaph after spraining her back.
It was due to be the monarch's first public outing after doctors advised her to rest for almost a month following medical checks in hospital.
In a statement, Buckingham Palace said: "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 Prince of Wales and Prime Minister Boris Johnson were among those to lay a wreath at the war memorial for the National Service of Remembrance, as the country fell silent for two minutes at 11am.
Mr Johnson said it was a moment to "come together to remember those who sacrificed everything in service of our country".
This year's event in Whitehall, central London, had added poignancy as it returned to its full size following the pandemic.
Labour leader Sir Keir Starmer and former prime ministers Tony Blair, Gordon Brown and Sir John Major also attended.
Sky's royal correspondent Rhiannon Mills said there had been a "huge amount of anticipation" ahead of the Queen's planned appearance at the service.
It is understood the monarch's back sprain is unrelated to the recent medical advice for her to rest.
Her injury is believed to have happened recently and there were concerns over the impact of a car journey and a period of standing.
A royal source said: "It is obviously incredibly unfortunate timing, and nobody regrets the Queen's absence today more deeply than Her Majesty herself."
The source added that the Queen is "deeply disappointed to miss the engagement which she regards as one of the most significant engagements of the year".
The palace had previously said it was the Queen's "firm intention" to attend the annual wreath-laying service in Whitehall.
The 95-year-old was due to watch the service from the balcony of the Foreign Office building.
Her attendance at the service had been announced on Thursday after she was ordered to rest by royal doctors just over three weeks ago. She had spent a night in hospital on 20 October undergoing tests.
Mills said the Remembrance Sunday service is particularly poignant for the Queen "because she is a member of the wartime generation".
"During the Second World War she saw her father rallying the nation as king," Mills said, adding that the Queen's late husband Prince Philip also served in the Royal Navy during the war.
It is understood the Queen - who will remain at Windsor Castle today - hopes to continue with her schedule of light official duties next week as planned.
Royal commentator Alastair Bruce said he expected the monarch was "immensely aggravated" to miss Sunday's service.
"She feels this is one of the principal roles that she performs," he told Sky News.
"For the Queen, she will have probably fretted about this - not least last night but certainly I expect this morning.
"And in making this decision she's probably working with the advice of her doctors.
"But she will be with the events that take place at the Cenotaph. She will watch it, she will play her part, and she will see her son - her heir apparent - lay down her wreath on behalf of the nation."
The Queen has only missed six other Cenotaph ceremonies during her reign: on four occasions when she was on overseas visits to Ghana in 1961, Brazil in 1968, Kenya in 1983 and South Africa in 1999.
She was not present during the 1959 and 1963 services as she was pregnant with her two youngest children.