Charles, his sister Princess Anne, and brothers Princes Andrew and Edward will join the ceremonial guard for the 15-minute vigil at Westminster Hall in central London on Friday, where their mother's coffin has rested since Wednesday evening.
Already tens of thousands of people from all walks of life from Britain and around the world have patiently queued for the opportunity to pass by the coffin. By Friday morning, the line stretched back almost 8km and people were facing a 14-hour wait to reach the hall.
The government said the queue, which snakes along the south side of the River Thames, was close to capacity and that entry may need to be paused.
"If you have not yet set off to join, please consider waiting until numbers have reduced," the culture department said.
Officials expect about 750,000 people to view the coffin before the lying-in-state ends at 6.30am on Monday, the day of Elizabeth's state funeral
Malcolm Keyte, 78, from Surrey, southern England, queued for nearly 10 hours with his two daughters.
"We just want to pay our respects and our thanks," he told Reuters near the front of the queue just after 7 a.m. (0600 GMT). "She was a wonderful lady who served the country very well, very loyally, for very many years and we should all be very grateful," he said.
Before the 7.30pm vigil, Charles and his wife Camilla, the Queen Consort, will travel to Wales.
The trip will mark the end of a tour of the UK where he has performed ceremonial duties to acknowledge his status as the new monarch and head of state, and to greet the public mourning the loss of Elizabeth, who reigned for 70 years and died last Thursday at the age of 96.
The royal couple will be greeted with a 21-gun salute, attend a cathedral service, and travel to the Welsh parliament. Charles will meet the Welsh first minister and other politicians.
Wales has a particular significance for the new king, who for five decades preceding last week's accession had the title Prince of Wales - longer than anyone previously.
Meanwhile the new Prince of Wales, Charles's son William, will visit troops from New Zealand, Canada and Australia who are in Britain to take part in events surrounding the state funeral.
He will be accompanied by his wife Kate, the new Princess of Wales. The title was previously held by William's mother Diana, killed in a car crash in 1997.
Her death likewise prompted a national outpouring of grief, and William spoke on Thursday of how the solemn events of the last week had revived memories of the funeral procession for Diana.
On Wednesday, William, alongside Charles and his younger brother Harry, walked in a procession behind a gun carriage carrying the Queen's coffin from Buckingham Palace to the Palace of Westminster, a scene highly reminiscent of when, as boys 25 years ago, the princes had followed Diana's casket.
"The walk yesterday was challenging ... brought back a few memories," William said as he and his wife Kate spoke to well-wishers and viewed the sea of floral tributes outside the royal residence of Sandringham in eastern England.
Britain is gearing up to welcome presidents, prime ministers and royalty from around the world for Monday's funeral, which is likely to be one of the grandest ceremonies ever seen in the British capital, involving thousands of military personnel.
