The Queen, seen at last year's State Opening of Parliament, will miss it for the first time in 59 years
This will be the first time since 1963 that the Queen will have missed this constitutional ceremony, which sets out the government's legislative plans.
Prince Charles will deliver the speech on Tuesday for the Queen.
The 96-year-old monarch has mobility problems and has had to cancel a number of recent public appearances.
Until Monday evening Buckingham Palace had been saying the Queen hoped to attend, but has now confirmed she will not go to the ceremony in Westminster, because of "episodic mobility problems".
A statement said the Queen, in consultation with her doctors, had reluctantly decided not to attend the State Opening.
Prince Charles and Prince William, the Duke of Cambridge, have jointly been given the authority to open Parliament on her behalf.
During her 70-year reign, the Queen has until now only missed two State Openings
The Imperial State Crown will still be brought to Parliament - and the Queen's throne will remain empty, with Prince Charles, Camilla, the Duchess of Cornwall, and Prince William expected to be seated in front of the assembled parliamentarians.
The Queen's absence follows her missing events at Easter, including the Maundy Service, and the announcement that she would not host royal garden parties this year.
The thanksgiving service for Prince Philip in March has been the only public event outside of royal residences that she has attended so far this year.
But it is understood that the Queen is planning to go ahead with other appointments this week, including regular meetings with the Prime Minister and Privy Council, held virtually or by phone, and some private engagements.
The State Opening of Parliament marks the start of the parliamentary year, with the Queen's speech setting out the agenda of the government and the laws that it wants to introduce.
The speech is usually read out by the monarch, as head of state, with the Queen only missing it twice during her 70-year reign, in 1959 and 1963, because of pregnancies.
On those occasions the speech was delivered by the Lord Chancellor, but the Prince of Wales will stand in for the Queen this year.
There have been adaptations of the State Opening in recent years - with the Queen not wearing the heavy Imperial State Crown or ceremonial robes and there was a more scaled-back ceremony last year because of Covid restrictions.
But this will mark the first time in 59 years that the Queen will not have attended.
Prince Charles will open Parliament with Prince William in their capacities as "counsellors of state", allowing them to undertake such official duties if the monarch is temporarily unwell.
Two of these counsellors have to be present to carry out this duty on behalf of the head of state.
There are four counsellors of state - the other two being Prince Andrew, who has stepped down from royal duties, and Prince Harry, who is also no longer carrying out royal duties and lives in the US.
The authority for the counsellors of state to open Parliament has been provided by a "Letters Patent", issued by the Queen and enabling Prince Charles and Prince William to carry out that role.
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."
-- Courtesy of BBC News