The two brothers will be joined by their six cousins in carrying out a 15-minute vigil at Westminster Hall in central London, a night after their father, King Charles, joined his three siblings in a similar tribute to the late Queen
Tens of thousands of people have already filed past the coffin in a steady, solemn stream, queuing for hours through cold temperatures, some through the night, to pay their respects to Britain's longest-reigning monarch - a testimony to the affection in which she was held.
"I'm here because of the Queen's service and dedication over the last 70 years," Reuben Fevrier, a 50-year-old accountant from London, said.
"Her stoicism, her deep commitment to the country - a remarkable life. This the least I can do."
On Friday, New Zealand Prime Minister Jacinda Ardern was seen curtsying to the coffin as she filed through - one of the first of nearly 100 presidents and heads of government who are expected to attend the state funeral in London on Monday.
London's police force said the funeral will be the biggest security operation it has ever undertaken.
Australia's Prime Minister Anthony Albanese, US President Joe Biden, French President Emmanuel Macron and the leaders of New Zealand, Canada and Jamaica will join the emperor of Japan among the congregation.
On Saturday, King Charles will meet the prime ministers of the 14 other countries where he is head of state.
He will also meet workers in the emergency services who are helping to stage the funeral.
Later the focus will switch to the younger royals and their vigil.
Heir-to-the-throne William and his brother Harry, who have grown apart in recent years after Harry moved to the United States, will both appear in military uniform.
Harry served two tours of duty with the British Army in Afghanistan but so far has appeared in processions in civilian dress after he lost his honorary military titles when he stepped back from public royal duties.
The two brothers will be joined at the vigil by their cousins - Peter Phillips and Zara Tindall, the children of Princess Anne; Princesses Beatrice and Eugenie, the children of Prince Andrew; and Louise and James, the children of Prince Edward.
Thousands of people continue to queue to see the Queen's coffin, with some facing more than a day in line.
As of 5am on Saturday (2pm AEST), the Department for Digital, Culture, Media & Sport's queue tracker said lines were expected to last for "at least 24 hours", after stretching out to 25 hours earlier in the morning.
Those inside Westminster Hall were briefly shocked on Friday night when a man was arrested after moving out of the queue to approach the Queen's coffin.
Two thousand people will gather inside Westminster Abbey on Monday for the Queen's funeral.
Some 800 people, including members of the Queen's Household and Windsor estate staff, will attend the committal service afterwards at 4pm (1am Tuesday AEST) in St George's Chapel, Windsor Castle.
After the funeral, the King and members of the royal family will walk behind the Queen's coffin to Wellington Arch when it leaves Westminster Abbey, before it is driven to Windsor on the state hearse.
In Australia, the Queen's funeral will be televised at 8pm AEST on Monday.
