Mourners in London wanting to see the Queen's coffin could expect to wait in line for up to 30 hours.
Queen Elizabeth II's coffin will leave Buckingham Palace for the last time as it is taken amid somber pageantry on a horse-drawn gun carriage past crowds of mourners to the historic Westminster Hall, where the late monarch will lie in state for four days until her funeral on September 19.
The Daily Mirror reported 750,000 mourning members of the public are expected to queue, with Culture Secretary Michelle Donelan telling Tory lawmakers in a group chat: "Queues could be up to 30 hours as we are obviously expecting and planning for unprecedented demand."
The New York Post has reported Palace officials have estimated the line to pay respects will be "miles long".
The line to see the coffin will be open 24 hours a day and the Mirror has reported there will be port-a-potties placed along the queue route – which is expected to stretch nearly 5km.
Mourners have been told they will need a wristband to enter the queue and will not be able to take large bags, food, and drinks or tributes into the palace.
Crowds began massing early along the flag-lined mall outside the palace for the procession from the monarch's official London residence to Westminster Hall at Parliament.
They are the latest manifestation of a nationwide outpouring of grief and respect for the only monarch most Britons have ever known, who died at her beloved Balmoral summer retreat on Thursday at age 96, ending a 70-year reign.
People stood behind metal barriers or sat on folding chairs, umbrellas at the ready, takeout coffees in hand under gray skies hours before the coffin was scheduled to leave the landmark palace at 2.22pm (1.22am tomorrow NZT).
Crowds have lined the route of the Queen's coffin whenever it has been moved in its long journey from Scotland back to London.
STORY CONTINUES AFTER BLOG
STORY CONTINUES
On Tuesday night, thousands braved a typical London drizzle as the state hearse, with interior lights illuminating the sovereign's flag-draped casket, drove slowly from a military air base into the heart of London.
Geoff Colgan, a taxi driver who took the day off to witness the moment, stood stunned in the moments after the queen's coffin passed.
"It's one of those things you know would happen, but when it does you can't believe it," he said, holding his toddler.
Earlier, in Edinburgh, some 33,000 people filed in silent respect past her coffin as it lay for 24 hours at St Giles' Cathedral.
Hundreds of thousands are expected to do the same in London when the Queen lies in state at the 900-year-old Westminster Hall, the oldest building in Parliament.
The hall is where Guy Fawkes and Charles I were tried, where kings and queens hosted magnificent medieval banquets, and where ceremonial addresses were presented to Queen Elizabeth II during her silver, golden and diamond jubilees.
Chris Bond, from Truro in southwest England, was among those lining up along the banks of the River Thames. He also attended the lying in state of the Queen Mother in 2002.
"Obviously, it's quite difficult queuing all day long, but when you walk through those doors into Westminster Hall, that marvellous, historic building, there was a great sense of hush and one was told you take as much time as you like, and it's just amazing," he said.
"We know the queen was a good age and she served the country a long time, but we hoped this day would never come," he added.