It is being widely reported that some of her children and grandchildren have already arrived at Balmoral with the rest on their way.
Many political commentators have said it is rare for Buckingham Palace to release such a statement about the Queen’s health.
The Queen was unable to attend a virtual Privy Council meeting on Wednesday evening.
What will happen if the Queen passes away in Balmoral?
The plan for what will happen should the Queen pass away in Scotland is called Operation Unicorn.
Once the monarch’s death is formally announced parliamentary business will be suspended for at least six parliamentary days.
The monarch’s coffin will then lie in repose at Holyrood Palace, followed by a service of reception at St. Giles’ Cathedral in Edinburgh.
The Royal Train will then take the Queen’s coffin from Waverly Station in Edinburgh to St. Pancras Station, London.
If this is not possible, the coffin will be taken to London by plane where it will be met by the prime minister and select secretaries of state.
The day the Queen dies will be called D-Day, with the following days called D+1, D+2 and so on.
The Prime Minister and the Privy Council will be informed of the Queen’s death by the monarch’s private secretary immediately.
Prince Charles will become king the moment the Queen dies, even if a proper coronation might not happen for months or even a year after the moment of her death. His official proclamation will happen on D+1.
Departmental permanent secretaries will be given a script to inform ministers of the government: “We have just been informed of the death of Her Majesty The Queen.” And ministers will be told that “discretion is required.”
After that, the cabinet will send a message to all civil servants, saying: “Dear colleagues, It is with sadness that I write to inform you of the death of Her Majesty The Queen.” And flags on Whitehall will be lowered to half-mast.
The first announcement to the media will go out from a major media organisation.
On the BBC, all programming across all their channels will be interrupted in order to show BBC One’s feed on the story.
What will happen to the Parliament?
Upon the Queen’s death, parliament, including those in Wales, Scotland and Northern Ireland will be adjourned.
The Prime Minister is the only one who is allowed to make a statement, and will do so as soon as possible.
Parliament will then have an audience with Prince Charles, now King Charles.
There will be a national minute’s silence followed by gun salutes organised by the Ministry of Defence, followed by a remembrance service at St Paul’s Cathedral in London.
What will happen with the Queen’s coffin and when will her funeral take place?
On D+4, the rehearsal of the procession from Buckingham Palace to the Palace of Westminster will take place, and the actual procession will take place the day after.
From D+5, on what is called Operation Feather, the Queen will rest in the Palace of Westminster for three days.
The funeral will most likely take place on D+10, ten days after the Queen’s death with a ceremony in Westminster Abbey.
This will be followed by a national two minute’s silence.
The Queen’s coffin will then be taken to Windsor Castle where she will be buried at the King George VI memorial chapel alongside her parents.