Charles III and his sons, Princes William and Harry, joined a sombre procession as the coffin of Queen Elizabeth II left Buckingham Palace in London on Wednesday, on its final journey to be conveyed to Westminster Hall in the Houses of Parliament complex for Lying-in-State until the state funeral at Westminster Abbey nearby on Monday.

At 2.22 pm local time, the coffin placed on a horse-drawn gun carriage of the King's Troop Royal Horse Artillery began its ceremonial procession of under two kilometres to the Palace of Westminster.

The Queen's heir, Charles, and her grandchildren William and Harry walked side by side behind the coffin, with gun salutes firing from Hyde Park and Big Ben tolling during a sombre journey.

The Queen's other children, Princess Anne and Princes Andrew and Edward, were also among the senior royals in the procession behind the carriage.

Thousands lined the route of the procession, which went past the Mall towards the Palace of Westminster by the river Thames.

The coffin will be received at Westminster Hall by the Archbishop of Canterbury, Most Reverend Justin Welby, to conduct a short service assisted by the Very Revd Dr David Hoyle, the Dean of Westminster. This ceremony will be joined by other members of the royal family as well before the coffin is placed on a catafalque, or raised platform.

As the Lying-in-State stage of the ceremony begins, a continuous vigil will be mounted by officers of the Household Division, the King's Bodyguards of the Honourable Corps of Gentlemen at Arms, The King's Bodyguard for Scotland, the Royal Company of Archers and the Yeomen of the Guard.

Westminster Hall will then open from 5 pm local time to the many thousands queuing since Monday evening to begin filing past the coffin to pay their respects to the late monarch.

Overnight on Tuesday, the coffin spent its final night in the Bow Room at Buckingham Palace for the most private part of the ongoing state mourning since the Queen passed away in Scotland last Thursday.

Earlier on Tuesday, crowds lined the streets of Edinburgh as a cortege took the coffin to the airport after lying in state at St. Giles' Cathedral in the Scottish capital. There was applause as the coffin left the cathedral, where thousands filed past since Monday to pay their respects to the late monarch.

The Scottish government said more than 33,000 people had paid their respects in Edinburgh before the coffin was taken by road to Edinburgh Airport. It was flown to London, accompanied by the Queen's daughter Princess Anne, on a Royal Air Force (RAF) aircraft that was used to evacuate thousands of people fleeing the Taliban in Kabul last year. The C-17 Globemaster has also been used to take humanitarian aid and weapons to Ukraine following Russia's invasion.

The aircraft landed at RAF Northolt before being taken to Buckingham Palace, where it was met by King Charles and Queen Consort Camilla as well as other members of the royal family. It was received by a guard of honour formed by the King's Guard before being taken to the Bow Room inside the palace to be watched overnight by a rota of chaplains.

Earlier on Tuesday, King Charles left Scotland for Northern Ireland as part of his Operation Spring Tide tour of state mourning to different parts of the United Kingdom. He visited Belfast for the first time as monarch and met Northern Ireland's political and religious leaders and took part in a prayer service at St. Anne's Cathedral.

Now, with that shining example before me, and with God's help, I take up my new duties resolved to seek the welfare of all the inhabitants of Northern Ireland. During the years of my mother's reign, it has been a privilege to bear witness to such a devoted life. May it be granted to us all to fulfil the tasks before us so well, he said in a short statement.

The King is scheduled to Wales at the end of the week, ahead of ceremonial events and audiences planned as world leaders who begin arriving in the UK for the state funeral at Westminster Abbey on Monday morning.

Queen Elizabeth II died aged 96 peacefully at her Balmoral Castle summer residence in Scotland on Thursday.