Prince William and Prince Harry will join King Charles III and walk behind Queen Elizabeth II's coffin from Buckingham Palace to Westminster Hall, where she will lie in state.
King Charles and the two princes — along with Prince Andrew, Princess Anne and Prince Edward — will follow the coffin on foot as it makes its journey on Wednesday afternoon.
Princess Anne’s son Peter Phillips and her husband, Vice Adm Sir Tim Laurence, will also walk in the procession, as well as Prince Richard, the Duke of Gloucester, and David Armstrong-Jones, the Earl of Snowdon.
Wednesday's funeral procession will inevitably have echoes of the moment, 25 years ago, when the young princes, then 15 and 12, walked behind their mother Princess Diana's coffin after her sudden death in a car accident in Paris.
The procession will leave the palace at 2.22pm, making its way along The Mall, Horse Guards Road, across Horse Guards Parade and on to Whitehall to Parliament Square and into the Palace of Westminster, arriving about 3pm.
A service lasting about 20 minutes will be led by the Archbishop of Canterbury, Justin Welby, accompanied by David Hoyle, the Dean of Westminster.
The doors to Westminster Hall will then be opened, allowing the public to file past to pay their respects to the queen from 5pm.
The hall will be open round the clock until early on Monday, the morning of the queen’s funeral.
Organisers expect thousands to attend to pay their respects, and have prepared for queues of up to eight kilometres in length.
They are warning people to prepare for a long and physically demanding wait, with few opportunities to sit down.
Everyone arriving at the hall will be issued with wristbands, which will allow them to pop out for a short time for food or drink.
The queen's coffin will be guarded round the clock until the funeral by members of the Household Division or Yeoman Warders of the Tower of London, who are the sovereign's bodyguards.
At moments, there was applause and cheers and a few blooms were thrown into the path of the state hearse, and when it neared Buckingham Palace three cheers went up, with people lighting up the area with the torches on their camera phones, holding them above their heads.
Rehearsals for the procession of Queen Elizabeth's coffin in London — in pictures
The brothers have a well-documented troubled relationship but the death of their grandmother led to them unexpectedly coming together to view floral tributes left to the late queen at Windsor Castle.
Prince William, Kate, Prince Harry and Meghan arrived in the same vehicle and greeted mourners for about 40 minutes before Prince William hopped into the driver’s seat of the Audi with his wife in the passenger seat, and his brother and sister-in-law in the back.
In his televised address to the nation on Friday evening, the king talked of his love for Prince Harry and Meghan, saying: “I want also to express my love for Harry and Meghan as they continue to build their lives overseas.”
The UK mourns Queen Elizabeth II — in pictures
Then in Prince Harry’s tribute to his grandmother, released on Monday, he said he wanted to honour his father at the start of his reign as king.
The last time King Charles and his two sons were all seen together in public was at the service of thanksgiving for the queen in St Paul’s Cathedral during the platinum jubilee celebrations in June.
But on that occasion, Prince Harry and Meghan were seated some distance from King Charles and Prince William, on the other side of the aisle in the second row, behind the Wessex family and the duke and duchess of Gloucester.
This was Prince Harry and Meghan’s first public appearance alongside the Windsors since they stepped down as working royal family members in 2020 during the Megxit storm.
Queen Elizabeth II's coffin arrives at Buckingham Palace — video
In April 2021, Prince Harry and Prince William joined their father when they walked behind the coffin of Prince Philip, the Duke of Edinburgh, at his funeral.
The brothers were separated by their cousin, Peter Phillips, but he dropped back half a pace at one point so the siblings appeared closer together.
After the funeral service, Prince William and Prince Harry could be seen chatting as they walked back up the hill from the chapel to the castle.