The monarch was due to deliver the Queen's Speech in Parliament tomorrow.
But a statement from the Palace reads: "The Queen continues to experience episodic mobility problems, and in consultation with her doctors has reluctantly decided that she will not attend the State Opening of Parliament tomorrow.
"At Her Majesty’s request, and with the agreement of the relevant authorities, The Prince of Wales will read The Queen’s speech on Her Majesty’s behalf, with The Duke of Cambridge also in attendance."
The Queen missing the event will be a first in 59 years.
Dominic Lipinski
Charles with the Queen at the State Opening of Parliament in 2019
Victoria Jones
A new Letters Patent authorised by the Queen was issued to cover the State Opening delegating to Counsellors of State the royal function of opening a new session of Parliament.
In this instance, it enables Charles and William to jointly exercise that function. No other functions have been delegated by the Queen.
The decision was taken today.
The episodic mobility issues are said to be a continuation of the problems the Queen has suffered since the autumn.
The Queen is understood to have a busy diary at Windsor this week with a call with Australia undertaken on Monday, and a planned virtual Privy Council and phone audience with the Prime Minister on Wednesday.
She is expected to undertake some private engagements later in the week.
A No 10 spokesman said: “The Prime Minister fully respects the wishes of Her Majesty and is grateful to the Prince of Wales for agreeing to deliver the speech on her behalf.”
The last occasion the Queen was unable to attend the Queen's Speech was in 1963, due to her being pregnant at the time.
The only other occasion the monarch was unable to attend was in 1959, for the same reason.
The news that the Queen will not be undertaking the duty this year means it will be the first time in 59 years it has occurred.
The Queen’s throne will remain empty in the House of Lords, and the Prince of Wales and the Duchess of Cornwall, who is also attending, will sit in their usual seats.
William will sit on the opposite side to Camilla.
The Imperial State Crown will still travel to Parliament.
Charles has issued a comment following the revelation, with a source saying: “The Prince of Wales is of course ready to support Her Majesty the Queen.”