The Queen will miss today's Remembrance Sunday service at the Cenotaph after spraining her back, Buckingham Palace has announced.
It was due to be the monarch's first public outing after being advised to rest by doctors 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."
It is understood the Queen's back sprain is unrelated to the recent medical advice for her to rest.
It is believed to be a recent injury 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.
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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".
It is understood she hopes to continue with her schedule of light official duties next week as planned.
A wreath will be laid on the Queen's behalf at the Cenotaph in London by Prince Charles.
He will be joined at the service by the Duchess of Cornwall and the Duke and Duchess of Cambridge.
The Queen hopes to continue as planned with her schedule of light official duties next week, a royal source said.