King Charles will be joined by his siblings to mount a silent vigil at the coffin of his late mother Queen Elizabeth on Friday as thousands of mourners stand in line for miles to pay their last respects to the monarch during her lying-in-state.
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 where their 96-year-old 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 hours for the opportunity to pass by the coffin, with the line stretching back almost 5 miles (8 km).
Officials expect about 750,000 people to view the coffin before the lying-in-state ends at 6.30 a.m. (0530 GMT) on Monday, the day of Elizabeth's state funeral.
Before the 7.30 p.m. (1830 GMT) vigil, Charles and his wife Camilla, the Queen Consort, will travel to Wales.
The trip will mark the end of a tour of the United Kingdom 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.
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.
"His passion and affection for Wales has been clear," his spokesman said. "He has shown a life-long commitment to the country's people."
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 Elizabeth's state funeral on Monday.
He will be accompanied by 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 sombre 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.
"It is our aim and belief that the state funeral and events of the next few days will unite people across the globe," said the Earl Marshal, Edward Fitzalan-Howard, the Duke of Norfolk, who is England's most senior peer and is in charge of state occasions.