The monarch, 96, reluctantly pulled out of the major ceremonial occasion as she continues to experience “episodic mobility problems” with royal doctors advising her against attending.
As Charles takes on the head of state’s major constitutional duty for the first time, the move – believed to be unprecedented in modern history – will be interpreted as a significant shift in his responsibilities as a king in waiting.
The Duke of Cambridge, also a future monarch, will also attend the State Opening – the first time William has done so.
Buckingham Palace said in a statement: “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.”
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 on Monday.
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.
It would be only the third time during her reign that the Queen has not opened parliament – and the first time nearly 60 years.
The exceptions were in 1959 and 1963, when she was pregnant with Prince Andrew and then Prince Edward, when her speech was read by the Lord Chancellor.