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Britain's Prince Charles, right, and Camilla, the Duchess of Cornwall, listen as Britain's Queen Elizabeth II delivers the Queen's Speech, in 2014.

King Charles and Camilla, now the Queen Consort, will remain at Balmoral on Thursday night and return to London on Friday.

"During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which the Queen was so widely held," said Charles' statement.

Buckingham Palace had confirmed on Thursday lunchtime that Queen Elizabeth II had been under medical supervision after her doctors had become "concerned" about her health.

Queen Elizabeth II and Prime Minister Jacinda Ardern pictured in 2018. Photo / Getty Images

All of the Queen's children, as well as Prince William, who assumes the position of heir to the throne and will now be known as the Duke of Cambridge and Cornwall, travelled immediately to be with her.

The Queen was last pictured formally appointing the new Prime Minister, Liz Truss, at Balmoral on Tuesday, her final public duty.

A lifetime of devotion

Queen Elizabeth II is survived by her four children, eight grandchildren and 12 great-grandchildren. With Charles now King, his eldest son, Prince William, the Duke of Cambridge, assumes the position of heir to the throne.

William and Kate's Twitter account @KensingtonRoyal now refers to the couple as The Duke and Duchess of Cornwall and Cambridge.

The Queen will be remembered for her selfless devotion to duty and the cause of the monarchy - which carried her through countless political and personal crises, often involving members of her own family.

Throughout it all - from the highs of her coronation and jubilee celebrations to the depths of her self-declared "annus horribilis" in 1992, when her children's marriages fell apart - her calm and dignified manner ensured strong public support throughout her long reign.

The Queen meets her great-grandson Archie Mountbatten-Windsor. Photo / AP

The Queen's death now triggers a series of events to allow her family and her subjects around the world to grieve.

Her body will be at Buckingham Palace for several days so the Royal family can grieve together.

After four days the Queen's coffin will be led on a military procession from the palace to Westminster Hall.

She will lie in state over the next four days, after which King Charles, the family, and dignitaries will pay their respects. Then, the doors will open to hundreds of thousands of people queued up outside.

The funeral is estimated to take place 10-12 days after the Queen's passing and will be attended by state officials from all over the world and will be co-ordinated by the armed forces and Government.

At 11am sharp the bells of Big Ben will chime, the country will fall silent, and the coffin will be brought inside Westminster Abbey, where 2000 specially invited guests will bow their heads in prayer.

After the service, the coffin will be taken to Windsor Castle, then finally to St George's Chapel, where Queen Elizabeth II will likely be laid to rest next to her father, King George VI.

King Charles III

Members of the royal family gather on the balcony of Buckingham Palace. Photo / AP

Queen Elizabeth's death leads to Charles ascending the throne. His wife Camilla will be known as Queen Consort, a request made by the Queen to mark the 70th anniversary of her reign in February 2022.

Charles is allowed to choose his own name - and is expected to become King Charles III.

Charles will be named King one day after the Queen's death - after his siblings have ceremoniously kissed his hand.

At 10am on the day after the Queen's death, the Accession Council — which includes senior government figures — meets at St James's Palace to proclaim King Charles the new sovereign.

Proclamations will be made and while the Queen lies in state, Charles will visit Scotland, Northern Ireland, and Wales. His first words as monarch are expected to take place at St James's Palace.

Elizabeth: A sovereign since the age of 25

Elizabeth Alexandra Mary Windsor came to the throne on the death of her father, King George VI, on February 6, 1952.

She was on a wildlife-watching escape in the central highlands of Kenya and days into a months-long Commonwealth tour, when her husband of four years, Prince Philip, broke the news of her father's death.

At the age of 25, she became the sovereign of almost 140 million people, welcomed onto the throne in a televised coronation that combined pomp and modernity.

She worked with 14 British prime ministers and met a dozen US presidents, four popes and countless other leaders, including Prime Minister Jacinda Ardern in 2018.

A frequent traveller until her last years, she undertook more than 270 overseas trips, and in 2011 became the first British monarch to visit the Republic of Ireland.

There were 10 visits to New Zealand and, as always, she favoured the brightly-coloured outfits she chose for all her public duties. Daughter-in-law, Sophie, Countess of Wessex explained why in 2016's Our Queen at Ninety.

"When she turns up somewhere, the crowds are two, three, four, ten, 15 deep, and someone wants to be able to say they saw a bit of the Queen's hat as she went past. She needs to stand out for people to be able to say, 'I saw the Queen'."

The Queen had four children, Charles, Anne, Andrew and Edward. She loved horses and corgis and bred both, branching out into "dorgis" with the help of a dachshund owned by her sister Princess Margaret.

Her approach to monarchy buoyed the public's perception of the royal family, with few rumblings of republicanism in the latter years of her reign, but the 1990s proved a low point - starting with her self-declared "annus horribilis" in 1992, when heir Charles and second-born son Andrew separated from their wives, daughter Anne divorced Mark Phillips and a major fire damaged Windsor Castle, one of her homes.

Scandals returned in recent years, notably the defection of her grandson Prince Harry and his American wife Meghan from "the Firm" to the United States and sexual abuse allegations against Andrew, a friend of convicted sex offender Jeffrey Epstein.

in April 2021 she lost her husband Prince Philip, the man she called "my strength and stay all these years", at the age of 99.

Queen's health declines

It was reported earlier that the Queen was under medical supervision in Balmoral, Scotland and her immediate family had travelled to be with her amid concerns about her health.

In a brief statement on Thursday night (NZ time), Buckingham Palace said the doctors of the 96-year-old monarch were "concerned for Her Majesty's health and have recommended she remain under medical supervision".

Her Majesty had earlier been advised by her doctors to rest after "a full day" appointing Liz Truss as the new British Prime Minister at Balmoral Castle, but following further evaluation the following morning, medical supervision was recommended.

The statement said the Queen "remains comfortable and at Balmoral" following the cancellation of her scheduled meeting with the Privy Council.

According to reports in the UK, members of the Royal Family have been informed of the Queen's condition, with several of them rushing to be with her.

Heir to the throne Prince Charles, and Camilla are staying nearby at Birkhall, according to Buckingham Palace.

The BBC reported a flight with seven members of the royal household - including Prince William, Prince Andrew and Prince Edward - had landed at Aberdeen just before 3am Friday (NZ time).

Kate, the Duchess of Cambridge, has remained in Windsor as Prince George, Princess Charlotte and Prince Louis are on their first full day at their new school, Kensington Palace has said.

The Duke and Duchess of Sussex, Harry and Meghan, are in the UK, and Harry will also be travelling to Balmoral, their spokesman has confirmed.

The pair are in the UK, with the Duke due to speak at the WellChild Awards in London, before returning to their children in the US.

Scores of well-wishers had gathered at Windsor Castle in Berkshire, while it was announced there would be no changing of the guard at Buckingham Palace.

The broadcaster had earlier cleared its viewing schedule, with interruptions to its regular viewing to break the news of the Palace announcement.

Frail, hunched over

The Queen is the longest-reigning British monarch and the second-longest reigning monarch in history after taking the crown in 1952.

Earlier this week, Her Majesty - looking frail and hunched over - was photographed when she formally appointed Truss.

Truss and outgoing prime minister Boris Johnson made the trip to Aberdeenshire for the key audiences this week.

Concerns were raised over what seemed to be a bruise on the back of the Queen's right hand, while she was also using a walking stick indoors, during her meeting with Truss.

The handover of power has historically taken place at Buckingham Palace, however, it was moved to Scotland in light of the Queen's mobility issues. It's the first time in her reign the location of the meeting has been changed.

The historic occasion marked the first time the Queen has been seen since she passed through Aberdeen Airport on her way to Balmoral in late July.

'Praying for the Queen's health'

Early on Friday morning (NZ time) Truss said: "The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime.

"My thoughts — and the thoughts of people across our United Kingdom — are with Her Majesty The Queen and her family at this time," she said on Twitter.

The UK's Speaker of the House of Commons, Sir Lindsay Hoyle, interrupted a speech to tell MPs about the Queen's health.

"I know I speak on behalf of the entire House when I say that we send our best wishes to Her Majesty the Queen and that she and the Royal Family are in our thoughts and prayers at this moment."

Truss also left the chamber as news broke, British political journalists reported.

In a tweet, the Archbishop of Canterbury said: "My prayers, and the prayers of people across the Church of England and the nation, are with Her Majesty The Queen today. May God's presence strengthen and comfort Her Majesty, her family, and those who are caring for her at Balmoral."

British MP David Lammy wrote on Twitter: "Incredibly concerned to hear reports from Buckingham Palace. I am praying for the Queen's health along with the rest of the country and millions around the world. Wishing her a full and speedy recovery."

In his first speech in the House of Commons, Business Secretary Jacob Rees-Mogg said "it is a matter of the gravest concern to all of us when our sovereign is unwell".

Meanwhile, political leaders from across the world have shared messages of support.

"All of us are feeling profoundly concerned at reports of Her Majesty's health," said Scotland's First Minister Nicola Sturgeon.

"My thoughts and wishes are with the Queen and all of the royal family at this time."

In a social media post, Wales' Mark Drakeford also shared his concern.

"Concerned to hear the news from Buckingham Palace. I send my best wishes to Her Majesty and her family on behalf of the people of Wales."

Speaking at a press conference in Cardiff some hours later, Drakeford said the news would bring "an afternoon of sober reflection and anxiety for households across Wales".

Former British PM David Cameron tweeted his "deep concern".

"I send my heartfelt thoughts and prayers to Her Majesty The Queen and the Royal Family at this worrying time," Cameron said.

Tony Blair said the update on the ailing Queen's health was "deeply concerning".

Palace officials have been forced to make decisions about the Queen's travel on a day-to-day basis in recent months.

Reports claim Prince Charles has been making "regular, daily" visits to his mother, which is seen as unusual.

The heir to the throne does not usually make unplanned visits to the Queen - they are most often arranged by palace officials.

Meanwhile, Prince Andrew has been staying with the Queen at Balmoral, which is where she spends most of her summers.

Her Majesty has pulled out from a number of planned events in recent times and had been resting for more than three months on medical advice.

Last year, she spent a night in hospital for "preliminary investigations" and did not attend the COP26 climate change summit in Glasgow. In April this year, she skipped the Maundy Thursday and Easter Matins church services at St George's Chapel in Windsor.

According to the Telegraph, the doctor tasked with her medical care is Professor Sir Huw Thomas - the Head of the Medical Household since 2014 and a professor of gastrointestinal genetics at Imperial College London's department of surgery and cancer.

The Queen's husband of 73 years, Prince Philip the Duke of Edinburgh, died at Windsor Castle in April last year. He was 99.

- Additional reporting AP