The Queen is under medical supervision at Balmoral Castle in Scotland, a statement from Buckingham Palace says
Her doctors are concerned for her health, but say the 96-year-old monarch "remains comfortable"
All of the Queen's four children and Prince William are now in Scotland to be with her - Prince Harry is on his way
Well-wishers are gathering at the gates of Balmoral, as well as outside Buckingham Palace, in London
New UK Prime Minister Liz Truss says "the whole country will be deeply concerned by the news"
The Archbishop of Canterbury says he is sending prayers and the first ministers of Scotland and Wales say the Queen is in their thoughts
Buckingham Palace has released a statement to say Queen Elizabeth II has died.
Our news report is here and we will be bringing you live updates here.
Andre Rhoden-Paul
Reporting from Buckingham Palace
Image source, Getty ImagesAs news has spread, the crowds have grown significantly outside Buckingham Palace.
Many people have come alone to take in the moment, while others have come in groups with friends.
Australian Chloe Hogan pictured with her two friends outside Buckinham Palace
Australian Chloe Hogan made her way to the palace after hearing the news about the Queen's health in her family group chat down. She says: "I think it's quite scary that we don't know what's going to happen." The Queen is also the monarch of Australia.
Chloe adds: "I think if anything did happen Australians would be just as devastated as when Princess Diana died, the exact same thing.
"It would be devastating, and everyone would be like people in the UK would be."
Facing the Queen Victoria monument Liran Shufman, from Italy, is joining dozens of people taking a picture of a rainbow.
The 40-year-old says the rainbow is hopefully sign that the Queen will return to good health.
Liran Shufman, from Italy takes a picture of the rainbow that appeared outside the palace
Image source, PA MediaQueen Elizabeth presented the George Cross to representatives of the National Health Service in July
The Queen's sense of duty is well known across the world, after seven decades as head of state.
But, as BBC royal correspondent Jonny Dymond explained earlier on BBC Radio 5 Live, most of us aren't aware of the "enormous number" of duties carried out by the Queen.
“Most of them happen below the surface – we don’t see them,” says Dymond.
“We might see the State Opening of Parliament or we might see her at Remembrance Day, but day-to-day there are literally hours of state duties.
"A lot of it is reading state papers - these great red boxes turn up at Buckingham Palace with all the state papers - she’s spent seven decades reading these things, she does it on her holidays, she did it when she was on royal tours."
He adds the Queen also receives incoming ambassadors, as well as carrying out a myriad of other duties include the Privy Council, which she had to miss last night on medical advice.
This is "a ceremonial part of state", says Dymond, "but an important one – part of what we are and what she has done for so long".
Sean Coughlan
BBC News, royal correspondent
Image source, ReutersPrince Charles took the Queen's place at the last State Opening of Parliament in May
The Queen has been struggling with her health for almost a year now.
Since last autumn she's had to pull out of a series of public events because of what seemed to be persistent problems with her mobility, often handing over duties to Prince Charles.
Remembrance Sunday, the Maundy Thursday service, the annual Commonwealth Service and the State Opening of Parliament have been among the events where health issues stopped the Queen from taking part over the past 12 months.
She rallied for her appearances on the balcony of Buckingham Palace during the Platinum Jubilee and only this week carried out her audience with the new prime minister.
Beyond many references to her mobility problems, Buckingham Palace officials have given little detail about changes in the Queen’s health, seeing it as a private matter.
But there has been no avoiding her advancing age and growing fragility, with a diminishing number of public appearances, and there are now fears of a further deterioration in her health.
Andre Rhoden-Paul
Reporting from Buckingham Palace
Around Buckingham Palace dozens of news cameras can be seen pointing at the Queen's London residence.
American tourist Judy Jones who is visiting with her sister told me she remembers watching the Queen's coronation in 1953 on a black and white TV when she was a girl.
"All my memory she has been Queen of England, it's very sad," she said.
Besides the tourists, Brits are also standing out in the rain outside the palace gates.
Londoner Holly Bell and her mum Rebecca Milner detoured from a gallery to the Palace after hearing the news about the Queen.
She says: "It feels like a historic moment, it felt important to be here. She's a national treasure."
Three floral bouquets have been laid outside the palace gates so far.
Steve Sheppard, who's been in London on business from Aberdeen, says: "Hopefully we don't see that flagpole going to half-mast. I really don't."
James Cook
Scotland Editor - Reporting from Balmoral
Image source, ReutersThere is little drama here at the gates of Balmoral. Little fuss.
Instead a well-rehearsed plan has swung quietly into action.
The police are directing traffic; and every few minutes another reporter or photographer crosses Isambard Kingdom Brunel's bridge over the River Dee to join the throng watching and waiting at the entrance of the estate.
For much of the afternoon the rain teemed down, leaving the purple heather sodden.
At 5.05pm there was a flash of camera bulbs as a motorcade swept past, with Prince William driving a car containing Princes Andrew, Edward; and Sophie, the Countess of Wessex.
They have come to a place the Queen loves, perhaps more than anywhere else.
Many people here in Royal Deeside return that affection.
In modern times Balmoral Castle, built in the 1850s for Queen Victoria, has been a constant in the life of the royal family - an enduring symbol of the monarchy.
Tonight thoughts and prayers here are with Victoria's great, great, granddaughter, Elizabeth.
Sean Coughlan
BBC News, royal correspondent
The Queen’s family are gathering around her, making their way from across the country to be with her at Balmoral.
It’s a reminder that these are sons, daughters, grandchildren and in-laws as well as public figures.
Prince William arrived a little while ago with his uncles Prince Andrew and Prince Edward and Prince Edward’s wife Sophie.
Kate, the Duchess of Cambridge, has remained in Windsor as her and William's children have had their first full day at a new school,
Prince Harry is on his way separately, but not accompanied by his wife Meghan, according to their spokeswoman.
Image source, PA MediaA Range Rover being driven by Prince William and carrying Prince Andrew, Prince Edward and Edward's wife Sophie has arrived at Balmoral Castle.
Image source, Getty ImagesRachel Bell
Reporting from Balmoral
Well-wishers have gathered at Balmoral, braving the heavy rainfall along with members of the media from around the world.
Several members of the Queen's immediate family are already at Balmoral to be by her side.
Her Majesty has been in residence at her beloved Scottish holiday home since July.
The statement released from Buckingham Palace earlier today said the Queen remains under medical supervision and that she is comfortable.
In the last few moments, the White House has said US President Joe Biden's thoughts are with the Queen and her family.
Prince Harry is travelling to Balmoral alone, according to his spokesperson.
It was reported earlier that he and his wife Meghan were on their way to Scotland together.
Canada's prime minister Justin Trudeau has tweeted well wishes to the Queen and the Royal Family.
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Image source, EPATourists have started gathering in the rain outside Buckingham Palace in London, following news that the Queen is under medical supervision at Balmoral
Image source, EPASome people have congregated on the stone steps up to The Queen Victoria Memorial, opposite the palace
Image source, PA MediaPeople are keeping across the latest news on their phones
Image source, EPAOthers are standing beside the gates
"The palace has gone further than they ever normally do," says the BBC's medical editor, Fergus Walsh.
He said the key phrase in the statement released by Buckingham Palace earlier, was "doctors were concerned".
He also highlighted the significance of family members arriving at Balmoral, where the Queen has been staying over the summer.
Until recently, her health has been "relatively robust" he explained.
He described the monarch as a "permanent fixture", adding that "we've all lived our lives with the Queen".
Ken Banks
Reporting from Balmoral
The rain is hammering down here at the gates of Balmoral.
A growing band of television cameras and reporters are gathering at the scene, as well as a small number of onlookers.
The mood is very muted, with people taking shelter under umbrellas and trees.
What chatter there is mainly revolves around news updates on the situation and who is arriving here.
A little earlier we heard from royal biographer Robert Hardman, author of Queen of Our Times.
He says today's news about the Queen's health has reminded us of her importance to the country and our affection for her.
"She is unlike any other monarch in our history – she’s our longest-lived, longest-serving, longest-reigning monarch," he tells BBC 5 Live.
He adds that her sense of public duty pre-dated her accession to the throne in 1952.
"She just stands for this constancy, this sense of permanence and stability.
"And I think over the years people have probably taken her for granted often.
"Suddenly, at times like this, we all realise... how precious she is."
Helena Wilkinson
Reporting from Windsor
Image source, Getty ImagesHere in Windsor the news is starting to trickle through.
The town, which is normally full of tourists, is quieter as it's been raining all afternoon.
But some of those who have ventured out to visit Windsor castle - the Queen's main residence - said they found out about the Queen's condition after overhearing castle staff talking about it.
Others found out as they left the grounds.
Some people from outside Windsor have also started travelling to this royal town.
One woman I spoke to had travelled from west London. She said after hearing the news, she wanted to be in Windsor.
Royal Family members arrive at Aberdeen airport
The plane that's landed in Aberdeen was carrying the Duke of Cambridge, the Duke of York and the Earl and Countess of Wessex, the BBC understands.
Figures were seen disembarking the plane and getting into cars on the airport runway. It's expected they will now travel to Balmoral Castle - a journey of around an hour.
Image source, PoolThe plane carrying seven members of the Royal Family has landed at Aberdeen airport.
It's not known who exactly is on the flight from RAF Northolt, although we know that Prince William, Prince Harry and Meghan, Prince Edward and Sophie, and Prince Andrew are all travelling to Balmoral.
Aberdeen airport is about an hour's drive from Balmoral Castle.
If you're just joining us, here's a reminder of where things stand amid concerns over the Queen's health.
Just after 12:30 this afternoon, Buckingham Palace issued a statement, external saying the Queen's doctors were concerned about her health, recommending she remained under medical supervision.
Soon after that, we heard news that all the Queen's children were either already with her at Balmoral Castle in Scotland or travelling to be with her.
Prince Charles, Camilla and Princess Anne are at Balmoral.
Prince William, the Queen's grandson and second in line to the throne, is on his way there, while his wife Kate, Duchess of Cambridge, has remained in Windsor as her children are on their first full day at their new school.
Prince William's brother, Prince Harry, and his wife Meghan had been in the UK in recent days for engagements, and are said to be travelling to Scotland too.
On Tuesday, newly appointed UK PM Liz Truss had herself travelled to Balmoral to be appointed by the Queen, in a break with tradition as the monarch would have normally seen her in London. But the Queen had suffered from mobility issues recently and so the meeting was moved to Balmoral.
We will bring you updates as we get them.