ITV News Presenter Mary Nightingale outlines this week's timetable of official events as the nation mourns the Queen's death
He automatically became King on the death of his mother, but the Accession Council, attended by Privy Councillors, confirmed his role on Saturday morning.
That period of royal mourning will be observed from now until seven days after the Queen’s funeral on Monday at Westminster Abbey.
The death of a monarch ushers in a series of events carefully choreographed over years of planning.
Here is the day-by-day account of what is expected to happen in the days ahead, leading up to the Queen's funeral on Monday 19 September, as Britain enters an official period of mourning.
Monday, 12 September
The House of Commons and the House of Lords are expected to come together in Westminster for a Motion of Condolence, which King Charles will attend and make a speech at.
Around 12.45pm the King and Queen Consort travel to Edinburgh. First they will inspect a Guard of Honour before attending the Ceremony of the Keys.
They will then join a procession from the Palace of Holyroodhouse to Saint Giles Cathedral. The King will then have an audience with the First Minister Nicola Sturgeon.
The procession is expected along the Royal Mile to St Giles’ Cathedral. There the Crown of Scotland will be placed on the coffin and there will be a service and the Vigil of the Princes by members of the royal family at 7.20pm.
The people of Scotland will be given a chance to pay their respects whilst the coffin lies in state at the Palace of Holyroodhouse. King Charles and the Queen Consort will spend the night in Scotland.
Tuesday, 13 September
King Charles will fly from Edinburgh to Belfast, heading to Hillsborough Castle when they land. There they will view an exhibition on the Queen's life.
Whilst in Belfast they will meet the Secretary of State for NI and Party Leaders. King Charles will receive messages of condolence and make his reply to them.
The King and Queen Consort will then visit St Anne’s Cathedral for a Service of prayer and reflection before later leaving Belfast for London.
At 6pm the Queen's coffin will be flown from Edinburgh Airport to RAF Northolt. It will be accompanied by Princess Ann and will land at 6.55pm when it will be carried to an awaiting state hearse.
The hearse will then depart for Buckingham Palace where its arrival will be witnessed by King Charles before resting in the Bow Room.
A rehearsal for the procession of the coffin from Buckingham Palace to the Palace of Westminster will also take place.
Wednesday, 14 September
The Queen's lying in state is expected to begin in Westminster Hall, in London - Operation Marquee - following a ceremonial procession through London.
The Archbishop of Canterbury will conduct a short service following the coffin's arrival. After the ceremony the King and Queen Consort will return to Buckingham palace by car.
From this point Lying-in-State will continue for four clear days, finishing at 6.30am on the morning of the state funeral on September, 19.
Hundreds of thousands of people will file past the coffin on its catafalque and pay their respects, just as they did for the Queen Mother's lying in state in 2002.
The management of the queues outside is codenamed Operation Feather.
Senior royals are also expected to pay their own moving tribute, standing guard at some stage around the coffin - the tradition known as the Vigil of the Princes.
Preparations are underway around Westminster to make way for the lengthy queues
During the Covid crisis, plans had been amended in case the Queen passed during the pandemic.
During that period, plans for lying in state had included the possibility of the introduction of timed ticketing for those wanting to attend.
Thursday, 15 September
Lying in state continues and a rehearsal is likely to take place for the state funeral procession.
Friday, 16 September - Sunday, 18 September
Lying in state continues, ending on Sunday. Heads of state begin to arrive for the funeral.
Monday, 19 September
The Queen's state funeral will take place at Westminster Abbey in central London at 11am.
The original plans are for the Queen's coffin to process on a gun carriage to the Abbey, pulled by naval ratings - sailors - using ropes rather than horses.
Senior members of the family are expected to poignantly follow behind - just like they did for the funeral of Diana, Princess of Wales, and for Prince Philip, the Duke of Edinburgh.
The military will line the streets and also join the procession.
Heads of state, prime ministers and presidents, European royals and key figures from public life will be invited to gather in the abbey, which can hold a congregation of 2,000.
The service will be televised, and a national two minutes' silence is expected to be held.
The same day as the funeral, the Queen's coffin will be taken to St George's Chapel at Windsor Castle for a televised committal service.
Later in the evening, there will be a private interment service with senior members of the royal family.
The Queen's final resting place will be the King George VI memorial chapel, an annex to the main chapel - where her mother and father were buried, alongside the ashes of her sister, Princess Margaret.
Philip's coffin will move from the Royal Vault to the memorial chapel to join the Queen's.
Where can you leave a tribute for the Queen?
Royal residences will close until after the Queen’s funeral. This includes The Queen's Gallery and the Royal Mews at Buckingham Palace, and The Queen's Gallery in Edinburgh.
Balmoral Castle and Sandringham House, the Queen's private estates, will also close for this period.
In addition, Hillsborough Castle, the Sovereign’s official residence in Northern Ireland, will be closed.
However the public can still offer tributes at dedicated sites, or sign an official condolences book for the Queen - which you can find here on the official Royal website.
Those who wish to leave flowers and notes at Buckingham Palace will be guided to lay them at dedicated sites in Green Park or Hyde Park, the Palace said.
Flowers left outside the Palace gates will be moved to the Green Park Floral Tribute Garden by The Royal Parks.
At Windsor Castle, floral tributes can be left at Cambridge Gate on the Long Walk. These flowers will be brought inside the castle each evening and placed on the castle chapter grass on the south side of St George’s Chapel and Cambridge Drive.
At the Sandringham Estate, people will be encouraged to leave floral tributes at the Norwich Gates.
At Balmoral Castle, floral tributes can be left at the main gate.
At the Palace of Holyroodhouse, people are encouraged to give floral tributes to the wardens at the entrance to The Queen’s Gallery. Those flowers will be laid on the forecourt grass in front of the Palace’s north turret.
At Hillsborough Castle, floral tributes may be laid on the castle forecourt in front of the main gates.
Here's what we saw in the hours and days immediately after the news broke from Balmoral of the Queen's death.
What has happened so far?
Day of Death
Thursday would traditionally have been D-Day or 'D+0' in the plans for the aftermath of the Queen's death, codenamed London Bridge.
But the announcement came late in the day - at around 6.30pm on Thursday, 8 September - meaning plans have been shifted a day to allow the complex arrangements to be put in place, meaning today will be considered D+0.
The King and Camilla - now the Queen Consort - remained at Balmoral overnight and returned to London on Friday.
Friday, 9 September
At 6pm, an official service for prayer and reflection was held at St Paul's Cathedral in London - about 2,000 members of the public attended, sitting alongside the Prime Minister and senior ministers.
The King's speech
King Charles made a televised address to the nation on Friday evening, having returned from Balmoral with Camilla, the Queen Consort.
He paid tribute to the Queen and pledged his duty to his service as the new sovereign.
What were King Charles' first duties as sovereign?
Audience with the Prime Minister: Despite his grief, duty calls for new sovereign King Charles III, who had his first audience as monarch with new Prime Minister Liz Truss.
Confirming funeral plans: King Charles was expected to meet the Earl Marshal - the Duke of Norfolk - who is in charge of the accession and the Queen's funeral, to approve the carefully choreographed schedule for the coming days.
'London Bridge': Arrangements have long been planned in consultation with the government, under the codename London Bridge. They will incorporate Operation Unicorn, the contingency plans for the death of the Queen in Scotland.
Court mourning: Charles III will decide on the length of court or royal mourning for members of the royal family and royal households. It is expected to last a month.
What flag traditions is Britain following?Union flags on royal buildings are flying at half-mast. Many public buildings, such as council town halls, are also lowering their flags out of respect.
The Royal Standard never flies half-mast. It represents the Sovereign and the United Kingdom, and is a symbol of the continuation of the monarchy.
If the new King is in residence at a royal palace or castle, the Royal Standard will fly there full-mast as is the tradition. The Union flag does not fly there at the same time.
Saturday, 10 September
The Accession Council met at St James's Palace in London to formally proclaim King Charles as the new sovereign.
First, the Privy Council gathered without the King to proclaim the new monarch and arranged business relating to the proclamation.
Then, Charles III held his first Privy Council, accompanied by Camilla - the new Queen Consort - and William who are also Privy Counsellors, and made his personal declaration and oath.
Union flags went to full-mast and remained there for 24 hours to coincide with proclamations around the country before returning to half-mast.
King Charles III also held audiences with the Prime Minister and the Cabinet.
The remarkable life of the Queen remembered in our latest episode of What You Need To Know