The public will be let in from 5:00 pm (1600 GMT), with mourners already warned they will face an endurance test in round-the-clock queues that could snake back up to five miles (eight kilometres).
“It’s going to be emotional and I don’t know how I’ll feel going in there as the first one,” said Vanessa Nanthakumaran, a 56-year-old administration assistant originally from Sri Lanka, who took her place at the head of the queue on Monday evening.
“It’s our duty to say thank you. It’s going to be prayers from the heart. It’s going to be very sombre, quiet and overwhelming.”
Strict rules and airport-style security measures have been put in place, with “far more” people expected than the 200,000 who filed past the coffin of the queen’s mother when she died in 2002, according to prime minister Liz Truss’s spokesman.
The government has advised people to wear “suitable clothing” and to bring portable battery packs to keep their mobile phones charged -- an indication that some people will need to wait overnight for a glimpse of the casket.
Hotel rooms in the capital are increasingly hard to find, with even budget rooms going for £300 ($350) per night, while transport bosses and police are under pressure to keep the city moving and safe in exceptional circumstances.
“It’s a massive challenge for the Metropolitan Police and for me personally, but we have been preparing for many, many years,” the newly appointed head of the London police force, Mark Rowley, told Sky News on Tuesday.
The body of the late 96-year-old monarch, who died “peacefully” at her Balmoral estate in Scotland last Thursday, was flown to London aboard an RAF plane on Tuesday evening from the Scottish capital Edinburgh.
The procession on Wednesday will mirror a similar ceremony in Edinburgh on Monday when her casket was driven through the hushed streets of the city to St. Giles’ Cathedral.
There, some 33,000 people filed past the coffin overnight to Tuesday afternoon, the Scottish government said.
“Scotland has now bid our Queen of Scots a sad, but fond farewell. We will not see her like again,” said first minister Nicola Sturgeon.
“It’s part of history... We’ll never see this again,” Lynn Templeton, visiting Edinburgh from northwest England, told AFP after filing past the coffin on Tuesday.
After Scotland and England, Charles continued his tour of the four nations of the United Kingdom on Tuesday by visiting Northern Ireland for the first time as king.
A trip to Wales is expected on Friday.
The 73-year-old new head of state has won wide praise in the British media for his dignified and often heartfelt reaction to his mother’s death which has led to a rare moment of public unity in Britain.
He has seen his popularity recover since the death of his former wife Diana in a 1997 car crash -- and his ratings have surged in recent days, according to a new survey on Tuesday.
The mourning has also obscured -- albeit briefly -- the broader country’s sharp political divisions and a severe cost-of-living crisis that is expected to cause a major increase in poverty over the winter.
The procession of the queen’s coffin will begin at precisely 2:22pm (1322 GMT) and will be attended by all of the queen’s siblings.
Big Ben will toll and artillery guns will fire salutes in Hyde Park.
The ceremony will also mean another prominent role for the queen’s scandal-hit son Andrew, who settled a case in the US earlier this year in which he was accused of sexually assaulting a 17-year-old.
Over the last week, he has made what is expected to be a brief return to the public eye during national mourning.
Not everyone shares the public mood of sadness and remembrance sparked by the queen’s death, with royal fatigue increasingly evident on social media in the face of blanket media coverage.
British police have also faced criticism from civil liberties groups over their treatment of anti-monarchy protesters who have publicly challenged Charles’ accession to the throne.
Video footage and witnesses have drawn attention to police arresting or intimidating people who shouted slogans against the monarchy or held up placards reading “Not My King”.
“Police officers have a duty to protect people’s right to protest as much as they have a duty to facilitate people’s right to express support, sorrow, or pay their respects,” campaign group Big Brother Watch said.
The queen’s funeral will take place in Westminster Abbey in front of 2,000 VIP guests, with the day declared a public holiday in Britain.
Hundreds of heads of state and government, as well as global royalty, are expected, but Russia, Belarus, Myanmar and North Korea have not been invited to send representatives.
US president Joe Biden has confirmed he will attend, as will French president Emmanuel Macron and Canadian prime minister Justin Trudeau.