Tens of thousands of mourners in central London will watch as the Queen's coffin is taken from Buckingham Palace to Parliament on Wednesday with the solemn procession led by the King and his sons William and Harry along with other senior royals.
The Queen's body will then lie in state in Westminster Hall until her funeral next Monday, September 19.
Her final journey began on Sunday from Balmoral, where she died, to Edinburgh and then her body remained for one final night at Buckingham Palace after she was taken to London on Tuesday.
A rota of chaplains will be watching over the coffin of the 96-year-old Queen in the Palace's Bow Room ahead of the procession on Wednesday afternoon.
The King and other royals will be present when the Imperial State Crown will be placed on top of the coffin along with a wreath of flowers and prayers will be read before it leaves Buckingham Palace.
At 2.22pm the coffin will be placed on a gun carriage of the King's Troop Royal Horse Artillery to be taken through central London to Westminster Hall, a medieval building with origins dating back to 1097 that is the oldest building on the parliamentary estate.
Charles will walk in silence behind the carriage with all of the other senior royals, including his siblings Anne, Andrew, and Edward.
Also in the procession will be William, 40, now the Prince of Wales, and Harry, 37.
But Kate and Meghan as well as Camilla are all going to be travelling by car to Parliament.
There are expected to be similar scenes in central London to what happened on the route from the airport to Buckingham Palace on Tuesday where tens of thousands grabbed any opportunity to grab a glimpse of the hearse despite the rain.
Much of central London will be closed to traffic and large crowds will be lining the route to see the coffin make its way while accompanied by guns firing every minute at Hyde Park and the tolling of Big Ben.
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, ahead of four days where the Queen's body will lie in state.
Those wishing to pay their respects will be able to file solemnly past the Queen’s coffin 24 hours a day from 5pm on Wednesday until 6.30am on September 19.
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.
A senior Palace official described Wednesday's event as relatively small and personal. The full-scale, ceremonial procession on the day of the day of her funeral, is likely to be one of the biggest the country has ever witnessed.
As many as 750,000 mourners are expected to file past the coffin during its period of lying-in-state, people have already begun queuing to pay their final respects, with the government warning they faced a long and arduous wait.
"This is a once-in-a-lifetime event to honour a once-in-a-lifetime woman," said Andrew Israels-Swenson from Minnesota, who said his 82-year-old British mother asked him to attend to "represent the family."
The Scottish government said about 33,000 people had filed past the coffin during the 24 hours it was at St Giles' Cathedral in Edinburgh, but the memorial in London is a much larger occasion.
The hundreds of thousands predicted to join the line will be asked to queue for up to 4.7 miles along the southern bank of the River Thames, winding past landmarks like the giant London Eye wheel and a reconstruction of Shakespeare's Globe theatre.
"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.
