King Charles, his sons Princes William and Harry and other senior royals will join a solemn procession as Queen Elizabeth's coffin is taken from Buckingham Palace to lie in state in parliament.
After the Queen's death last week at her summer home of Balmoral Castle in Scotland, her coffin was taken to Edinburgh for a series of ceremonies , and was flown to London late on Tuesday.
Tens of thousands of people lined the 22km route in driving rain and cars stopped on normally busy roads to see the well-lit hearse driving slowly through the dark towards Buckingham Palace, drawing applause and cheers.
At the palace, the flag-draped coffin was met by Charles and all the Queen's children, grandchildren and their spouses who had gathered together for the first time since their matriarch died.
"It has been an honour and a privilege to accompany her on her final journeys," said Elizabeth's daughter Anne, 72, who flew from Scotland alongside the coffin. "Witnessing the love and respect shown by so many on these journeys has been both humbling and uplifting."
The Queen's death, at the age of 96, has plunged the nation into mourning for a monarch who had reigned for 70 years.
People started queuing in the streets late on Tuesday to be one of the first to file past the coffin when the official lying in state period begins later on Wednesday.
"She really did hold up her oath of doing everything she could for this country," said Veronica Lewis, 52, from Worthing, southern England.
On Wednesday, the Imperial State Crown will be placed on top of the coffin, along with a wreath, at Buckingham Palace.
From there it will move at 2.22pm on a gun carriage of the King's Troop Royal Horse Artillery through central London to Westminster Hall, a medieval building with origins dating back to 1097 that is the oldest on the parliamentary estate.
Charles will walk in silence behind the carriage with other senior royals, including his siblings Anne, Andrew and Edward.
Also in the procession will be his sons William, 40, now the Prince of Wales, and Harry, 37, the Duke of Sussex, whose once close relationship has deteriorated in recent years to such an extent that they were said to barely be on speaking terms.
However, they appeared together with their wives when they met well-wishers outside Windsor Castle on Saturday, in a show of unity which hinted at a possible rapprochement.
Kate, William's wife and now the Princess of Wales, and Harry's wife, Meghan, will travel by car, as will Charles' wife Camilla, now the Queen Consort.
With much of central London closed to traffic, large crowds were building along the route to watch Wednesday's procession, which will be accompanied by guns firing every minute at Hyde Park, while parliament's Big Ben bell will toll.
When the cortege reaches Westminster Hall at the Palace of Westminster, the coffin will be carried inside by soldiers from the Grenadier Guards and placed on a catafalque. There will be a short service conducted by the Archbishop of Canterbury, the spiritual head of the Anglican Church.
Four days of lying in state will then begin until the funeral on September 19.
The full-scale ceremonial procession on the day of her funeral, is likely to be one of the biggest the country has ever witnessed.
With as many as 750,000 mourners expected to file past the coffin during its period of lying in state, people queued and slept on the streets in the rain to be able to pay their final respects.
The government has warned the queue could stretch for up to almost 8km along the southern bank of the River Thames.
"You will need to stand for many hours, possibly overnight, with very little opportunity to sit down as the queue will be continuously moving," the government said.
"She's an icon of icons," mourner Chris Imafidon said. "I must at least endure this camping out of respect."